diff --git a/dissect/cstruct/types/structure.py b/dissect/cstruct/types/structure.py
index 6c67516..ef8927b 100644
--- a/dissect/cstruct/types/structure.py
+++ b/dissect/cstruct/types/structure.py
@@ -324,6 +324,12 @@ def _calc_size_and_offsets(self) -> None:
             size = max(len(field.type), size)
             alignment = max(field.alignment, alignment)
 
+        if self.align and size is not None:
+            # Add "tail padding" if we need to align
+            # This bit magic rounds up to the next alignment boundary
+            # E.g. offset = 3; alignment = 8; -offset & (alignment - 1) = 5
+            size += -size & (alignment - 1)
+
         self.size = size
         self.alignment = alignment
 
diff --git a/tests/test_align.py b/tests/test_align.py
index 7e92a2b..859a879 100644
--- a/tests/test_align.py
+++ b/tests/test_align.py
@@ -72,6 +72,21 @@ def test_align_union():
     assert obj.dumps() == buf
 
 
+def test_align_union_tail():
+    d = """
+    union test {
+        uint64 a;
+        uint32 b[3];
+    };
+    """
+    c = cstruct.cstruct()
+    c.load(d, align=True)
+
+    assert c.test.align
+    assert c.test.alignment == 8
+    assert c.test.size == 16
+
+
 def test_align_array():
     d = """
     struct test {