diff --git a/.vscode/python.code-snippets b/.vscode/python.code-snippets
index 90b102510..4c7885278 100644
--- a/.vscode/python.code-snippets
+++ b/.vscode/python.code-snippets
@@ -21,7 +21,7 @@
 				"	entity = ${4:Person}",
 				"	definition_period = ${5:YEAR}",
 				"	value_type = ${6:float}",
-				"	unit = \"${7:currency-GBP}\"",
+				"	unit = ${7:GBP}",
 				"",
 				""
 		 	],
diff --git a/changelog_entry.yaml b/changelog_entry.yaml
index e69de29bb..c78b8f1dd 100644
--- a/changelog_entry.yaml
+++ b/changelog_entry.yaml
@@ -0,0 +1,4 @@
+- bump: minor
+  changes:
+    changed:
+    - Validated and standardised National Insurance variables.
diff --git a/policyengine_uk/data/datasets/__init__.py b/policyengine_uk/data/datasets/__init__.py
index 81b51924e..0f0cdf573 100644
--- a/policyengine_uk/data/datasets/__init__.py
+++ b/policyengine_uk/data/datasets/__init__.py
@@ -15,6 +15,7 @@
     CalibratedSPIEnhancedPooledFRS_2018_20,
     CalibratedSPIEnhancedPooledFRS_2019_21,
     EnhancedFRS,
+    UKMOD_FRS_2018,
 )
 
 DATASETS = [
@@ -34,4 +35,5 @@
     CalibratedSPIEnhancedPooledFRS_2018_20,
     CalibratedSPIEnhancedPooledFRS_2019_21,
     EnhancedFRS,
+    UKMOD_FRS_2018,
 ]
diff --git a/policyengine_uk/data/datasets/frs/__init__.py b/policyengine_uk/data/datasets/frs/__init__.py
index 816709241..01bbc8317 100644
--- a/policyengine_uk/data/datasets/frs/__init__.py
+++ b/policyengine_uk/data/datasets/frs/__init__.py
@@ -22,3 +22,4 @@
 )
 from .stacked_frs import StackedFRS, PooledFRS_2018_20, PooledFRS_2019_21
 from .enhanced_frs import EnhancedFRS
+from .ukmod import UKMOD_FRS_2018
diff --git a/policyengine_uk/data/datasets/frs/ukmod.py b/policyengine_uk/data/datasets/frs/ukmod.py
new file mode 100644
index 000000000..1dfdac0c8
--- /dev/null
+++ b/policyengine_uk/data/datasets/frs/ukmod.py
@@ -0,0 +1,63 @@
+import pandas as pd
+from policyengine_uk.data.storage import STORAGE_FOLDER
+import numpy as np
+from policyengine_core.data import Dataset
+
+
+class UKMOD_FRS_2018(Dataset):
+    name = "ukmod_frs_2018"
+    label = "UKMOD (2018-19 FRS)"
+    data_format = Dataset.TIME_PERIOD_ARRAYS
+    file_path = STORAGE_FOLDER / "ukmod_frs_2018.h5"
+    time_period = "2018"
+
+    def generate(self):
+        data = {}
+        ukmod_output = pd.read_csv(
+            STORAGE_FOLDER / "uk_2018_std.txt", delimiter="\t"
+        )
+        ukmod_input = pd.read_csv(
+            STORAGE_FOLDER / "uk_2018_a4.txt", delimiter="\t"
+        )
+        output_columns = [
+            column
+            for column in ukmod_output.columns
+            if column not in ukmod_input.columns
+        ]
+        ukmod = pd.merge(
+            ukmod_output[output_columns + ["idperson"]],
+            ukmod_input,
+            on="idperson",
+            how="right",
+        )
+        # Add ID variables first
+        data["person_id"] = ukmod.idperson
+        data["person_benunit_id"] = person_benunit_id = (
+            ukmod.idorigbenunit * 10 + ukmod.idorighh
+        )
+        data["person_household_id"] = person_household_id = (
+            ukmod.idorighh * 100
+        )
+        data["person_state_id"] = np.ones_like(ukmod.idperson)
+
+        data["benunit_id"] = person_benunit_id.unique()
+        data["household_id"] = person_household_id.unique()
+        data["state_id"] = np.array([1])
+
+        data["age"] = ukmod.dag.values
+        data["gender"] = np.where(
+            ukmod.dgn == 0,
+            "FEMALE",
+            "MALE",
+        ).astype("S")
+        data["employment_income"] = ukmod.yem.values * 12
+        data["self_employment_income"] = ukmod.yse.values * 12
+        data["pension_income"] = ukmod.ypp.values * 12
+        data["statutory_sick_pay"] = ukmod.bhlwk.values * 12
+        data["statutory_maternity_pay"] = ukmod.bmact_s.values * 12
+        data["statutory_paternity_pay"] = ukmod.bpact_s.values * 12
+
+        for variable in data:
+            data[variable] = {"2018": data[variable]}
+
+        self.save_dataset(data)
diff --git a/policyengine_uk/data/storage/__init__.py b/policyengine_uk/data/storage/__init__.py
new file mode 100644
index 000000000..111a9735f
--- /dev/null
+++ b/policyengine_uk/data/storage/__init__.py
@@ -0,0 +1,3 @@
+from pathlib import Path
+
+STORAGE_FOLDER = Path(__file__).parent
diff --git a/policyengine_uk/parameters/gov/dwp/state_pension/female_age.yaml b/policyengine_uk/parameters/gov/dwp/state_pension/female_age.yaml
index 02be4b75c..cdef4ca9c 100644
--- a/policyengine_uk/parameters/gov/dwp/state_pension/female_age.yaml
+++ b/policyengine_uk/parameters/gov/dwp/state_pension/female_age.yaml
@@ -5,7 +5,7 @@ values:
   2012-01-01: 62
   2014-01-01: 63
   2016-01-01: 64
-  2018-01-01: 65
+  2019-01-01: 65
   2020-01-01: 66
 metadata:
   unit: year
diff --git a/policyengine_uk/parameters/gov/hmrc/national_insurance/class_1/thresholds/secondary_threshold.yaml b/policyengine_uk/parameters/gov/hmrc/national_insurance/class_1/thresholds/secondary_threshold.yaml
index d4bac076a..0371cf12f 100644
--- a/policyengine_uk/parameters/gov/hmrc/national_insurance/class_1/thresholds/secondary_threshold.yaml
+++ b/policyengine_uk/parameters/gov/hmrc/national_insurance/class_1/thresholds/secondary_threshold.yaml
@@ -8,7 +8,6 @@ metadata:
   - https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/882271/Table-a4.pdf
   - https://www.gov.uk/government/publications/rates-and-allowances-national-insurance-contributions/rates-and-allowances-national-insurance-contributions
   unit: currency-GBP
-  uprating: gov.benefit_uprating_cpi
 values:
   2015-04-06: 155
   2016-04-06: 155
@@ -24,3 +23,4 @@ values:
         title: The Social Security (Contributions) (Rates, Limits and Thresholds Amendments
           and National Insurance Funds Payments) Regulations 2022(6)
     value: 175
+  2024-01-01: 175
\ No newline at end of file
diff --git a/policyengine_uk/tests/microsimulation/test_against_ukmod.py b/policyengine_uk/tests/microsimulation/test_against_ukmod.py
new file mode 100644
index 000000000..12bfc0ce4
--- /dev/null
+++ b/policyengine_uk/tests/microsimulation/test_against_ukmod.py
@@ -0,0 +1,66 @@
+from policyengine_uk import Microsimulation
+from policyengine_uk.data.datasets import UKMOD_FRS_2018
+from policyengine_uk.data.storage import STORAGE_FOLDER
+import pandas as pd
+import numpy as np
+import pytest
+
+SKIP_UKMOD_TESTS = True
+
+if not SKIP_UKMOD_TESTS:
+    ukmod_output = pd.read_csv(
+        STORAGE_FOLDER / "uk_2018_std.txt", delimiter="\t"
+    )
+    ukmod_input = pd.read_csv(
+        STORAGE_FOLDER / "uk_2018_a4.txt", delimiter="\t"
+    )
+    output_columns = [
+        column
+        for column in ukmod_output.columns
+        if column not in ukmod_input.columns
+    ]
+    ukmod = pd.merge(
+        ukmod_output[output_columns + ["idperson"]],
+        ukmod_input,
+        on="idperson",
+        how="right",
+    )
+
+    UKMOD_FRS_2018().generate()
+    sim = Microsimulation(dataset="ukmod_frs_2018")
+
+
+@pytest.mark.skip(reason="UKMOD data not publicly shareable")
+def test_ni_class_1():
+    # NI Class 1 income matches.
+    assert np.allclose(
+        sim.calculate("ni_class_1_income").values,
+        ukmod.il_empniearns.values * 12,
+        atol=1,
+    )
+
+
+@pytest.mark.skip(reason="UKMOD data not publicly shareable")
+def test_ni_class_1_employee():
+    # NI contributions are off by more because the thresholds change mid-year,
+    # and PolicyEngine simulates over the full year while UKMOD simulates one
+    # month.
+    assert np.allclose(
+        sim.calculate("ni_class_1_employee").values,
+        ukmod.tscee_s.values * 12,
+        atol=50,
+    )
+
+
+@pytest.mark.skip(reason="UKMOD data not publicly shareable")
+def test_ni_self_employed():
+    # NI self-employed contributions don't match entirely for people with both
+    # self-employed and employment income. This might be due to a different
+    # interpretation of the rules around capped NI contributions (our Class 4
+    # maximum uses the legislation as a reference).
+
+    error = np.abs(
+        sim.calculate("ni_self_employed").values - ukmod.tscse_s.values * 12
+    )
+
+    assert (error < 50).mean() > 0.99
diff --git a/policyengine_uk/tests/policy/baseline/finance/tax/national_insurance/class_1.yaml b/policyengine_uk/tests/policy/baseline/finance/tax/national_insurance/class_1.yaml
deleted file mode 100644
index c807ee196..000000000
--- a/policyengine_uk/tests/policy/baseline/finance/tax/national_insurance/class_1.yaml
+++ /dev/null
@@ -1,22 +0,0 @@
-- name: NI Class 1 for earner
-  period: 2021
-  absolute_error_margin: 10
-  input:
-    employment_income: 30000
-  output:
-    employee_NI_class_1: 2460
-- name: NI Class 1 for earner in 2019
-  period: 2019
-  absolute_error_margin: 30
-  input:
-    employment_income: 30000
-  output:
-    employee_NI_class_1: 2589
-- name: NI Class 1 for no earnings
-  period: 2019
-  absolute_error_margin: 0
-  input:
-    employment_income: 0
-  output:
-    employee_NI_class_1: 0
-    employer_NI_class_1: 0
diff --git a/policyengine_uk/tests/policy/baseline/finance/tax/national_insurance/class_2.yaml b/policyengine_uk/tests/policy/baseline/finance/tax/national_insurance/class_2.yaml
deleted file mode 100644
index 047f15817..000000000
--- a/policyengine_uk/tests/policy/baseline/finance/tax/national_insurance/class_2.yaml
+++ /dev/null
@@ -1,21 +0,0 @@
-- name: NI Class 2 for trader
-  period: 2020
-  absolute_error_margin: 10
-  input:
-    self_employment_income: 30000
-  output:
-    NI_class_2: 159
-- name: NI Class 2 for no income
-  period: 2020
-  absolute_error_margin: 0
-  input:
-    self_employment_income: 0
-  output:
-    NI_class_2: 0
-- name: NI Class 2 for income under limit
-  period: 2020
-  absolute_error_margin: 0
-  input:
-    self_employment_income: 5000
-  output:
-    NI_class_2: 0
diff --git a/policyengine_uk/tests/policy/baseline/finance/tax/national_insurance/class_4.yaml b/policyengine_uk/tests/policy/baseline/finance/tax/national_insurance/class_4.yaml
deleted file mode 100644
index b314a093f..000000000
--- a/policyengine_uk/tests/policy/baseline/finance/tax/national_insurance/class_4.yaml
+++ /dev/null
@@ -1,21 +0,0 @@
-- name: NI Class 4 for no income
-  period: 2020
-  absolute_error_margin: 0
-  input:
-    self_employment_income: 0
-  output:
-    NI_class_4: 0
-- name: NI Class 4 for income under upper limit
-  period: 2020
-  absolute_error_margin: 10
-  input:
-    self_employment_income: 50000
-  output:
-    NI_class_4: 3723
-- name: NI Class 4 for income over upper limit
-  period: 2020
-  absolute_error_margin: 10
-  input:
-    self_employment_income: 60000
-  output:
-    NI_class_4: 3923
diff --git a/policyengine_uk/tests/policy/baseline/finance/tax/national_insurance/national_insurance.yaml b/policyengine_uk/tests/policy/baseline/finance/tax/national_insurance/national_insurance.yaml
deleted file mode 100644
index 64d92f871..000000000
--- a/policyengine_uk/tests/policy/baseline/finance/tax/national_insurance/national_insurance.yaml
+++ /dev/null
@@ -1,7 +0,0 @@
-- name: Trader with some income
-  period: 2020
-  absolute_error_margin: 10
-  input:
-    self_employment_income: 34003
-  output:
-    national_insurance: 2439
diff --git a/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_employee.yaml b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_employee.yaml
new file mode 100644
index 000000000..d8cfd51ff
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_employee.yaml
@@ -0,0 +1,22 @@
+- name: NI Class 1 employee contributions - below PT
+  period: 2023
+  input:
+    ni_class_1_income: 11_000
+  output:
+    ni_class_1_employee: 0
+
+- name: NI Class 1 employee contributions - between PT and UEL
+  period: 2023
+  absolute_error_margin: 1
+  input:
+    ni_class_1_income: 30_000
+  output:
+    ni_class_1_employee: (30_000 - 12_570) * 0.12
+
+- name: NI Class 1 employee contributions - above UEL
+  period: 2023
+  absolute_error_margin: 1
+  input:
+    ni_class_1_income: 70000
+  output:
+    ni_class_1_employee: (50_270 - 12_570) * 0.12 + (70_000 - 50_270) * 0.0325
diff --git a/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_employee_additional.yaml b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_employee_additional.yaml
new file mode 100644
index 000000000..c3132fff8
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_employee_additional.yaml
@@ -0,0 +1,15 @@
+- name: NI Class 1 employee additional contributions - income below UEL
+  period: 2023
+  absolute_error_margin: 1
+  input:
+    ni_class_1_income: 30_000
+  output:
+    ni_class_1_employee_additional: 0
+
+- name: NI Class 1 employee additional contributions - income above UEL
+  period: 2023
+  absolute_error_margin: 1
+  input:
+    ni_class_1_income: 100_000
+  output:
+    ni_class_1_employee_additional: (100_000 - 50_270) * 0.0325
diff --git a/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_employee_primary.yaml b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_employee_primary.yaml
new file mode 100644
index 000000000..47bdd17f4
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_employee_primary.yaml
@@ -0,0 +1,7 @@
+- name: £20k income has NI Class 1 liability
+  period: 2023
+  absolute_error_margin: 1
+  input:
+    employment_income: 20_000
+  output:
+    ni_class_1_employee_primary: (20_000 - 12_570) * 0.12
\ No newline at end of file
diff --git a/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_employer.yaml b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_employer.yaml
new file mode 100644
index 000000000..7b73d917e
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_employer.yaml
@@ -0,0 +1,15 @@
+- name: NI Class 1 employer contributions for low income in 2023
+  period: 2023
+  absolute_error_margin: 0.01
+  input:
+    ni_class_1_income: 8_000  # Annual income below secondary threshold
+  output:
+    ni_class_1_employer: 0.00  # Expected employer contributions
+
+- name: NI Class 1 employer contributions for moderate income in 2023
+  period: 2023
+  absolute_error_margin: 0.01
+  input:
+    ni_class_1_income: 30_000  # Annual income above secondary threshold but below upper limit
+  output:
+    ni_class_1_employer: (30_000 - 175 * 52) * 0.138  # Expected employer contributions
diff --git a/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_income.yaml b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_income.yaml
new file mode 100644
index 000000000..abeae1edc
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_class_1_income.yaml
@@ -0,0 +1,9 @@
+- name: NI Class 1 income sums income components.
+  period: 2023
+  input:
+    employment_income: 1
+    statutory_sick_pay: 2
+    statutory_maternity_pay: 4
+    statutory_paternity_pay: 8
+  output:
+    ni_class_1_income: 15
diff --git a/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_liable.yaml b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_liable.yaml
new file mode 100644
index 000000000..a0834d81c
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_1/ni_liable.yaml
@@ -0,0 +1,20 @@
+- name: Child isn't liable for NI.
+  period: 2023
+  input:
+    age: 15
+  output:
+    ni_liable: false
+
+- name: Working-age adult is liable for NI.
+  period: 2023
+  input:
+    age: 35
+  output:
+    ni_liable: true
+
+- name: Retired adult isn't liable for NI.
+  period: 2023
+  input:
+    age: 70
+  output:
+    ni_liable: false
diff --git a/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_2/ni_class_2.yaml b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_2/ni_class_2.yaml
new file mode 100644
index 000000000..3a458e7fa
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_2/ni_class_2.yaml
@@ -0,0 +1,13 @@
+- name: NI Class 2 - under LPL.
+  period: 2023
+  input:
+    self_employment_income: 5_000
+  output:
+    ni_class_2: 0
+
+- name: NI Class 2 - over LPL.
+  period: 2023
+  input:
+    self_employment_income: 15_000
+  output:
+    ni_class_2: 3.15 * 52
diff --git a/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_4/ni_class_4.yaml b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_4/ni_class_4.yaml
new file mode 100644
index 000000000..c1892ae91
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_4/ni_class_4.yaml
@@ -0,0 +1,7 @@
+- name: NI Class 4 - under UPL.
+  period: 2023
+  absolute_error_margin: 1
+  input:
+    self_employment_income: 30_000
+  output:
+    ni_class_4: 1628
\ No newline at end of file
diff --git a/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_4/ni_class_4_main.yaml b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_4/ni_class_4_main.yaml
new file mode 100644
index 000000000..c4189d2a6
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_4/ni_class_4_main.yaml
@@ -0,0 +1,7 @@
+- name: NI Class 4 - over UPL has maximum capped.
+  period: 2023
+  absolute_error_margin: 1
+  input:
+    self_employment_income: 100_000
+  output:
+    ni_class_4_main: 3452
diff --git a/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_4/ni_class_4_maximum.yaml b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_4/ni_class_4_maximum.yaml
new file mode 100644
index 000000000..ca5693c7b
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_4/ni_class_4_maximum.yaml
@@ -0,0 +1,8 @@
+- name: NI Class 4 with employment and self-employment income - over UPL has maximum capped.
+  period: 2023
+  absolute_error_margin: 1
+  input:
+    self_employment_income: 100_000
+    employment_income: 100_000
+  output:
+    ni_class_4_maximum: 48_661
\ No newline at end of file
diff --git a/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/national_insurance.yaml b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/national_insurance.yaml
new file mode 100644
index 000000000..1bea92e94
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/national_insurance.yaml
@@ -0,0 +1,9 @@
+- name: National Insurance sums non-employer-side contributions.
+  period: 2023
+  input:
+    ni_class_1_employee: 1
+    ni_class_2: 2
+    ni_class_3: 4
+    ni_class_4: 8
+  output:
+    national_insurance: 15
diff --git a/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/ni_employee.yaml b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/ni_employee.yaml
new file mode 100644
index 000000000..bf9e244ed
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/ni_employee.yaml
@@ -0,0 +1,6 @@
+- name: NI employee sums Class 1 contributions.
+  period: 2023
+  input:
+    ni_class_1_employee: 1
+  output:
+    ni_employee: 1
\ No newline at end of file
diff --git a/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/ni_self_employed.yaml b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/ni_self_employed.yaml
new file mode 100644
index 000000000..c1728e1de
--- /dev/null
+++ b/policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/ni_self_employed.yaml
@@ -0,0 +1,7 @@
+- name: NI self-employed sums Class 2 and 4 contributions.
+  period: 2023
+  input:
+    ni_class_2: 1
+    ni_class_4: 2
+  output:
+    ni_self_employed: 3
\ No newline at end of file
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/class_1.py b/policyengine_uk/variables/gov/hmrc/national_insurance/class_1.py
deleted file mode 100644
index e7b38649b..000000000
--- a/policyengine_uk/variables/gov/hmrc/national_insurance/class_1.py
+++ /dev/null
@@ -1,99 +0,0 @@
-from policyengine_uk.model_api import *
-
-
-class NI_exempt(Variable):
-    value_type = bool
-    entity = Person
-    label = "Exempt from National Insurance"
-    documentation = "Whether a person is exempt from National Insurance"
-    definition_period = YEAR
-    reference = "Social Security Contributions and Benefits Act 1992 s. 6"
-
-    def formula(person, period, parameters):
-        return ~person("over_16", period) | person("is_SP_age", period)
-
-
-class monthly_employee_NI_class_1(Variable):
-    value_type = float
-    entity = Person
-    label = "Monthly employee Class 1 Contributions for National Insurance"
-    definition_period = MONTH
-    reference = "Social Security Contributions and Benefits Act 1992 s. 8"
-    unit = GBP
-
-    def formula(person, period, parameters):
-        class_1 = parameters(period).gov.hmrc.national_insurance.class_1
-        earnings = (
-            person("employment_income", period.this_year) / MONTHS_IN_YEAR
-        )
-        main_earnings = amount_between(
-            earnings,
-            class_1.thresholds.primary_threshold
-            * WEEKS_IN_YEAR
-            / MONTHS_IN_YEAR,
-            class_1.thresholds.upper_earnings_limit
-            * WEEKS_IN_YEAR
-            / MONTHS_IN_YEAR,
-        )
-        add_earnings = max_(
-            earnings
-            - class_1.thresholds.upper_earnings_limit
-            * WEEKS_IN_YEAR
-            / MONTHS_IN_YEAR,
-            0,
-        )
-        main_charge = class_1.rates.employee.main * main_earnings
-        add_charge = class_1.rates.employee.additional * add_earnings
-        return main_charge + add_charge
-
-
-class employee_NI_class_1(Variable):
-    label = "Employee NI class 1"
-    entity = Person
-    definition_period = YEAR
-    value_type = float
-    unit = "currency-GBP"
-
-    def formula(person, period, parameters):
-        total_NI = 0
-        for month in period.get_subperiods("month"):
-            total_NI += person("monthly_employee_NI_class_1", month)
-        return total_NI
-
-
-class employer_NI_class_1(Variable):
-    value_type = float
-    entity = Person
-    label = "Employer Class 1 Contributions for National Insurance"
-    definition_period = YEAR
-    reference = "Social Security Contributions and Benefits Act 1992 s. 8"
-    unit = GBP
-
-    def formula(person, period, parameters):
-        class_1 = parameters(period).gov.hmrc.national_insurance.class_1
-        earnings = person("employment_income", period)
-        main_earnings = max_(
-            earnings - class_1.thresholds.secondary_threshold * WEEKS_IN_YEAR,
-            0,
-        )
-        return class_1.rates.employer * main_earnings
-
-
-class employer_NI(Variable):
-    value_type = float
-    entity = Person
-    label = "Employer contributions to National Insurance"
-    definition_period = YEAR
-    unit = GBP
-
-    adds = ["employer_NI_class_1"]
-
-
-class total_NI(Variable):
-    value_type = float
-    entity = Person
-    label = "National Insurance (total)"
-    definition_period = YEAR
-    unit = GBP
-
-    adds = ["employer_NI", "national_insurance"]
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_employee.py b/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_employee.py
new file mode 100644
index 000000000..6440582c7
--- /dev/null
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_employee.py
@@ -0,0 +1,15 @@
+from policyengine_uk.model_api import *
+
+
+class ni_class_1_employee(Variable):
+    label = "NI Class 1 employee-side contributions"
+    entity = Person
+    definition_period = MONTH
+    value_type = float
+    unit = GBP
+    defined_for = "ni_liable"
+    adds = [
+        "ni_class_1_employee_primary",
+        "ni_class_1_employee_additional",
+    ]
+    reference = "https://www.legislation.gov.uk/ukpga/1992/4/section/8"
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_employee_additional.py b/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_employee_additional.py
new file mode 100644
index 000000000..eb23397d3
--- /dev/null
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_employee_additional.py
@@ -0,0 +1,23 @@
+from policyengine_uk.model_api import *
+
+
+class ni_class_1_employee_additional(Variable):
+    label = "NI Class 1 employee-side additional contributions"
+    entity = Person
+    definition_period = MONTH
+    value_type = float
+    unit = GBP
+    reference = "https://www.legislation.gov.uk/ukpga/1992/4/section/8"
+
+    def formula(person, period, parameters):
+        income = person("ni_class_1_income", period)
+        parameters = parameters(period).gov.hmrc.national_insurance.class_1
+        upper_earnings_limit = (
+            parameters.thresholds.upper_earnings_limit
+            * WEEKS_IN_YEAR
+            / MONTHS_IN_YEAR
+        )
+        upper_earnings_limit_income = max_(income - upper_earnings_limit, 0)
+        return (
+            parameters.rates.employee.additional * upper_earnings_limit_income
+        )
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_employee_primary.py b/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_employee_primary.py
new file mode 100644
index 000000000..e54c987e1
--- /dev/null
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_employee_primary.py
@@ -0,0 +1,33 @@
+from policyengine_uk.model_api import *
+
+
+class ni_class_1_employee_primary(Variable):
+    label = "NI Class 1 employee-side primary contributions"
+    entity = Person
+    definition_period = MONTH
+    value_type = float
+    unit = GBP
+    reference = "https://www.legislation.gov.uk/ukpga/1992/4/section/8"
+
+    def formula(person, period, parameters):
+        income = person("ni_class_1_income", period)
+        parameters = parameters(period).gov.hmrc.national_insurance.class_1
+
+        # Thresholds are weekly, so multiply by weeks in year and divide by months in year
+        primary_threshold = (
+            parameters.thresholds.primary_threshold
+            * WEEKS_IN_YEAR
+            / MONTHS_IN_YEAR
+        )
+        upper_earnings_limit = (
+            parameters.thresholds.upper_earnings_limit
+            * WEEKS_IN_YEAR
+            / MONTHS_IN_YEAR
+        )
+
+        upper_earnings_limit_income = max_(income - upper_earnings_limit, 0)
+        primary_threshold_income = (
+            max_(income - primary_threshold, 0) - upper_earnings_limit_income
+        )
+
+        return parameters.rates.employee.main * primary_threshold_income
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_employer.py b/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_employer.py
new file mode 100644
index 000000000..4e763c8bc
--- /dev/null
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_employer.py
@@ -0,0 +1,25 @@
+from policyengine_uk.model_api import *
+
+
+class ni_class_1_employer(Variable):
+    value_type = float
+    entity = Person
+    label = "NI Class 1 employer-side contributions"
+    definition_period = MONTH
+    unit = GBP
+    defined_for = "ni_liable"
+    reference = "https://www.legislation.gov.uk/ukpga/1992/4/section/9"
+
+    def formula(person, period, parameters):
+        class_1 = parameters(period).gov.hmrc.national_insurance.class_1
+        earnings = person("ni_class_1_income", period)
+        secondary_threshold = (
+            class_1.thresholds.secondary_threshold
+            * WEEKS_IN_YEAR
+            / MONTHS_IN_YEAR
+        )
+        main_earnings = max_(
+            earnings - secondary_threshold,
+            0,
+        )
+        return class_1.rates.employer * main_earnings
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_income.py b/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_income.py
new file mode 100644
index 000000000..f79e81fba
--- /dev/null
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_class_1_income.py
@@ -0,0 +1,17 @@
+from policyengine_uk.model_api import *
+
+
+class ni_class_1_income(Variable):
+    label = "ni_class_1_income"
+    documentation = "Income subject to NI Class 1 contributions."
+    entity = Person
+    definition_period = YEAR
+    value_type = float
+    unit = GBP
+    adds = [
+        "employment_income",
+        "statutory_sick_pay",
+        "statutory_maternity_pay",
+        "statutory_paternity_pay",
+    ]
+    reference = "https://www.legislation.gov.uk/ukpga/1992/4/section/3"
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_liable.py b/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_liable.py
new file mode 100644
index 000000000..656ba8ca1
--- /dev/null
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/class_1/ni_liable.py
@@ -0,0 +1,14 @@
+from policyengine_uk.model_api import *
+
+
+class ni_liable(Variable):
+    label = "NI liable"
+    documentation = (
+        "Whether this person is liable for NI contributions by their age."
+    )
+    entity = Person
+    definition_period = YEAR
+    value_type = bool
+
+    def formula(person, period, parameters):
+        return person("over_16", period) & ~person("is_SP_age", period)
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/class_2.py b/policyengine_uk/variables/gov/hmrc/national_insurance/class_2/ni_class_2.py
similarity index 61%
rename from policyengine_uk/variables/gov/hmrc/national_insurance/class_2.py
rename to policyengine_uk/variables/gov/hmrc/national_insurance/class_2/ni_class_2.py
index 9dbf1374c..a36358bb4 100644
--- a/policyengine_uk/variables/gov/hmrc/national_insurance/class_2.py
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/class_2/ni_class_2.py
@@ -1,26 +1,18 @@
 from policyengine_uk.model_api import *
 
 
-class weekly_NI_class_2(Variable):
+class ni_class_2(Variable):
     value_type = float
     entity = Person
-    label = "Class 2 Contributions for National Insurance"
+    label = "NI Class 2 contributions"
     definition_period = YEAR
     reference = "Social Security and Benefits Act 1992 s. 11"
     unit = GBP
+    defined_for = "ni_liable"
+    reference = "https://www.legislation.gov.uk/ukpga/1992/4/section/11"
 
     def formula(person, period, parameters):
         class_2 = parameters(period).gov.hmrc.national_insurance.class_2
         profits = person("self_employment_income", period)
         over_threshold = profits >= class_2.small_profits_threshold
         return over_threshold * class_2.flat_rate * WEEKS_IN_YEAR
-
-
-class NI_class_2(Variable):
-    value_type = float
-    entity = Person
-    label = "Class 2 Contributions for National Insurance for the year"
-    definition_period = YEAR
-    unit = GBP
-
-    adds = ["weekly_NI_class_2"]
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/class_3/ni_class_3.py b/policyengine_uk/variables/gov/hmrc/national_insurance/class_3/ni_class_3.py
new file mode 100644
index 000000000..860049e4b
--- /dev/null
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/class_3/ni_class_3.py
@@ -0,0 +1,12 @@
+from policyengine_uk.model_api import *
+
+
+class ni_class_3(Variable):
+    label = "NI Class 3 contributions"
+    documentation = "Voluntary contributions to NI."
+    entity = Person
+    definition_period = YEAR
+    value_type = float
+    unit = GBP
+    defined_for = "ni_liable"
+    reference = "https://www.legislation.gov.uk/ukpga/1992/4/section/13"
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/class_4.py b/policyengine_uk/variables/gov/hmrc/national_insurance/class_4.py
deleted file mode 100644
index 48f0ecef1..000000000
--- a/policyengine_uk/variables/gov/hmrc/national_insurance/class_4.py
+++ /dev/null
@@ -1,63 +0,0 @@
-from policyengine_uk.model_api import *
-
-
-class NI_class_4(Variable):
-    value_type = float
-    entity = Person
-    label = "Class 4 Contributions for National Insurance for the year"
-    definition_period = YEAR
-    reference = "Social Security and Benefits Act 1992 s. 15"
-    unit = GBP
-
-    def formula(person, period, parameters):
-        class_4 = parameters(period).gov.hmrc.national_insurance.class_4
-        self_employment_income = person("self_employment_income", period)
-        employee_NI = person("employee_NI", period)
-        profits = self_employment_income - employee_NI
-        main_amount = amount_between(
-            profits,
-            class_4.thresholds.lower_profits_limit,
-            class_4.thresholds.upper_profits_limit,
-        )
-        add_amount = max_(profits - class_4.thresholds.upper_profits_limit, 0)
-        main_charge = main_amount * class_4.rates.main
-        add_charge = add_amount * class_4.rates.additional
-        return main_charge + add_charge
-
-
-class employee_NI(Variable):
-    value_type = float
-    entity = Person
-    label = "employee-side National Insurance"
-    definition_period = YEAR
-    unit = GBP
-
-    def formula(person, period, parameters):
-        exempt = person("NI_exempt", period)
-        return person("employee_NI_class_1", period) * ~exempt
-
-
-class self_employed_NI(Variable):
-    value_type = float
-    entity = Person
-    label = "self-employed National Insurance"
-    definition_period = YEAR
-    unit = GBP
-
-    def formula(person, period, parameters):
-        exempt = person("NI_exempt", period)
-        return add(person, period, ("NI_class_2", "NI_class_4")) * ~exempt
-
-
-class national_insurance(Variable):
-    value_type = float
-    entity = Person
-    label = "National Insurance"
-    documentation = "Total National Insurance contributions"
-    definition_period = YEAR
-    unit = GBP
-    reference = "Social Security and Benefits Act 1992 s. 1(2)"
-    adds = [
-        "employee_NI",
-        "self_employed_NI",
-    ]
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/class_4/ni_class_4.py b/policyengine_uk/variables/gov/hmrc/national_insurance/class_4/ni_class_4.py
new file mode 100644
index 000000000..90b0282ee
--- /dev/null
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/class_4/ni_class_4.py
@@ -0,0 +1,36 @@
+from policyengine_uk.model_api import *
+
+
+class ni_class_4(Variable):
+    value_type = float
+    entity = Person
+    label = "NI Class 4 main contributions"
+    definition_period = YEAR
+    unit = GBP
+    defined_for = "ni_liable"
+
+    def formula(person, period, parameters):
+        class_4 = parameters(period).gov.hmrc.national_insurance.class_4
+        self_employment_income = person("self_employment_income", period)
+        employee_NI = person("ni_class_1_employee", period)
+        profits = self_employment_income - employee_NI
+        add_rate_income = max_(
+            profits - class_4.thresholds.upper_profits_limit,
+            0,
+        )
+        main_rate_income = (
+            max_(
+                profits - class_4.thresholds.lower_profits_limit,
+                0,
+            )
+            - add_rate_income
+        )
+        pre_maximum_amount = (
+            main_rate_income * class_4.rates.main
+            + add_rate_income * class_4.rates.additional
+        )
+        maximum_amount = person("ni_class_4_maximum", period)
+        return max_(
+            min_(pre_maximum_amount, maximum_amount),
+            0,
+        )
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/class_4/ni_class_4_main.py b/policyengine_uk/variables/gov/hmrc/national_insurance/class_4/ni_class_4_main.py
new file mode 100644
index 000000000..3b1fbac5f
--- /dev/null
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/class_4/ni_class_4_main.py
@@ -0,0 +1,29 @@
+from policyengine_uk.model_api import *
+
+
+class ni_class_4_main(Variable):
+    value_type = float
+    entity = Person
+    label = "NI Class 4 main contributions"
+    definition_period = YEAR
+    unit = GBP
+    defined_for = "ni_liable"
+    reference = "https://www.legislation.gov.uk/ukpga/1992/4/section/15"
+
+    def formula(person, period, parameters):
+        class_4 = parameters(period).gov.hmrc.national_insurance.class_4
+        self_employment_income = person("self_employment_income", period)
+        employee_NI = person("ni_class_1_employee", period)
+        profits = self_employment_income - employee_NI
+        add_rate_income = max_(
+            profits - class_4.thresholds.upper_profits_limit,
+            0,
+        )
+        main_rate_income = (
+            max_(
+                profits - class_4.thresholds.lower_profits_limit,
+                0,
+            )
+            - add_rate_income
+        )
+        return main_rate_income * class_4.rates.main
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/class_4/ni_class_4_maximum.py b/policyengine_uk/variables/gov/hmrc/national_insurance/class_4/ni_class_4_maximum.py
new file mode 100644
index 000000000..426126a99
--- /dev/null
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/class_4/ni_class_4_maximum.py
@@ -0,0 +1,48 @@
+from policyengine_uk.model_api import *
+
+
+class ni_class_4_maximum(Variable):
+    label = "NI Class 4 maximum liability"
+    entity = Person
+    definition_period = YEAR
+    value_type = float
+    unit = GBP
+    reference = "https://www.legislation.gov.uk/uksi/2001/1004/regulation/100"
+
+    def formula(person, period, parameters):
+        ni = parameters(period).gov.hmrc.national_insurance
+        upl = ni.class_4.thresholds.upper_profits_limit
+        lpl = ni.class_4.thresholds.lower_profits_limit
+        step_1 = upl - lpl
+        main_rate = ni.class_4.rates.main
+        add_rate = ni.class_4.rates.additional
+        step_2 = step_1 * main_rate
+        step_3 = step_2 + 53 * ni.class_2.flat_rate
+        class_2_contributions = person("ni_class_2", period)
+        primary_class_1_contributions = person(
+            "ni_class_1_employee_primary", period
+        )
+        step_4 = step_3 - class_2_contributions - primary_class_1_contributions
+        class_4_main_contributions = person("ni_class_4_main", period)
+        other_aggregate_contributions = (
+            primary_class_1_contributions
+            + class_2_contributions
+            + class_4_main_contributions
+        )
+        case_1 = (step_4 >= 0) & (step_4 > other_aggregate_contributions)
+        case_2 = (step_4 >= 0) & (step_4 <= other_aggregate_contributions)
+        case_3 = step_4 < 0
+        step_5 = step_4 * 100 / 9
+        profits = person("self_employment_income", period)
+        step_6 = lpl - min_(upl, profits)
+        step_7 = max_(0, step_6 - step_5)
+        step_8 = step_7 * add_rate
+        step_9 = max_(0, profits - upl)
+
+        return select(
+            [
+                case_1,
+                case_2 | case_3,
+            ],
+            [step_4, step_4 + step_8 + step_9],
+        )
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/national_insurance.py b/policyengine_uk/variables/gov/hmrc/national_insurance/national_insurance.py
new file mode 100644
index 000000000..429a27730
--- /dev/null
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/national_insurance.py
@@ -0,0 +1,17 @@
+from policyengine_uk.model_api import *
+
+
+class national_insurance(Variable):
+    value_type = float
+    entity = Person
+    label = "National Insurance"
+    documentation = "Total National Insurance contributions"
+    definition_period = YEAR
+    unit = GBP
+    reference = "Social Security and Benefits Act 1992 s. 1(2)"
+    adds = [
+        "ni_class_1_employee",
+        "ni_class_2",
+        "ni_class_3",
+        "ni_class_4",
+    ]
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/ni_employee.py b/policyengine_uk/variables/gov/hmrc/national_insurance/ni_employee.py
new file mode 100644
index 000000000..a4ccedace
--- /dev/null
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/ni_employee.py
@@ -0,0 +1,12 @@
+from policyengine_uk.model_api import *
+
+
+class ni_employee(Variable):
+    value_type = float
+    entity = Person
+    label = "employee-side National Insurance"
+    definition_period = YEAR
+    unit = GBP
+    adds = [
+        "ni_class_1_employee",
+    ]
diff --git a/policyengine_uk/variables/gov/hmrc/national_insurance/ni_self_employed.py b/policyengine_uk/variables/gov/hmrc/national_insurance/ni_self_employed.py
new file mode 100644
index 000000000..9924062d5
--- /dev/null
+++ b/policyengine_uk/variables/gov/hmrc/national_insurance/ni_self_employed.py
@@ -0,0 +1,13 @@
+from policyengine_uk.model_api import *
+
+
+class ni_self_employed(Variable):
+    value_type = float
+    entity = Person
+    label = "self-employed National Insurance"
+    definition_period = YEAR
+    unit = GBP
+    adds = [
+        "ni_class_2",
+        "ni_class_4",
+    ]
diff --git a/policyengine_uk/variables/household/income/income.py b/policyengine_uk/variables/household/income/income.py
index 36b4aa097..4a87d2318 100644
--- a/policyengine_uk/variables/household/income/income.py
+++ b/policyengine_uk/variables/household/income/income.py
@@ -355,3 +355,27 @@ class income_decile(Variable):
 
     def formula(person, period, parameters):
         return person.household("household_income_decile", period)
+
+
+class statutory_maternity_pay(Variable):
+    label = "Statutory maternity pay"
+    entity = Person
+    definition_period = YEAR
+    value_type = float
+    unit = GBP
+
+
+class statutory_paternity_pay(Variable):
+    label = "Statutory paternity pay"
+    entity = Person
+    definition_period = YEAR
+    value_type = float
+    unit = GBP
+
+
+class statutory_sick_pay(Variable):
+    label = "Statutory sick pay"
+    entity = Person
+    definition_period = YEAR
+    value_type = float
+    unit = GBP
diff --git a/policyengine_uk/variables/input/income.py b/policyengine_uk/variables/input/income.py
index 8dacf08e6..54a6b6e4f 100644
--- a/policyengine_uk/variables/input/income.py
+++ b/policyengine_uk/variables/input/income.py
@@ -51,7 +51,7 @@ class self_employment_income(Variable):
     value_type = float
     entity = Person
     label = "self-employment income"
-    documentation = "Income from self-employment profits"
+    documentation = "Income from self-employment profits. This should be net of self-employment expenses."
     definition_period = YEAR
     unit = GBP
     reference = "Income Tax (Trading and Other Income) Act 2005 s. 1(1)(a)"