diff --git a/lib/std/fmt/parse_float.zig b/lib/std/fmt/parse_float.zig
index 854108db5eef..7e22477617bb 100644
--- a/lib/std/fmt/parse_float.zig
+++ b/lib/std/fmt/parse_float.zig
@@ -1,4 +1,4 @@
-const std = @import("../std.zig");
+const std = @import("std");
 const math = std.math;
 const testing = std.testing;
 const expect = testing.expect;
@@ -151,6 +151,12 @@ test "#11169" {
     try expectEqual(try parseFloat(f128, "9007199254740993.0"), 9007199254740993.0);
 }
 
+test "many_digits hex" {
+    const a: f32 = try std.fmt.parseFloat(f32, "0xffffffffffffffff.0p0");
+    const b: f32 = @floatCast(try std.fmt.parseFloat(f128, "0xffffffffffffffff.0p0"));
+    try std.testing.expectEqual(a, b);
+}
+
 test "hex.special" {
     try testing.expect(math.isNan(try parseFloat(f32, "nAn")));
     try testing.expect(math.isPositiveInf(try parseFloat(f32, "iNf")));
diff --git a/lib/std/fmt/parse_float/FloatStream.zig b/lib/std/fmt/parse_float/FloatStream.zig
index ad31d1591909..6f9ba89e54b6 100644
--- a/lib/std/fmt/parse_float/FloatStream.zig
+++ b/lib/std/fmt/parse_float/FloatStream.zig
@@ -48,30 +48,16 @@ pub fn isEmpty(self: FloatStream) bool {
     return !self.hasLen(1);
 }
 
-pub fn firstIs(self: FloatStream, c: u8) bool {
+pub fn firstIs(self: FloatStream, comptime cs: []const u8) bool {
     if (self.first()) |ok| {
-        return ok == c;
+        inline for (cs) |c| if (ok == c) return true;
     }
     return false;
 }
 
-pub fn firstIsLower(self: FloatStream, c: u8) bool {
+pub fn firstIsLower(self: FloatStream, comptime cs: []const u8) bool {
     if (self.first()) |ok| {
-        return ok | 0x20 == c;
-    }
-    return false;
-}
-
-pub fn firstIs2(self: FloatStream, c1: u8, c2: u8) bool {
-    if (self.first()) |ok| {
-        return ok == c1 or ok == c2;
-    }
-    return false;
-}
-
-pub fn firstIs3(self: FloatStream, c1: u8, c2: u8, c3: u8) bool {
-    if (self.first()) |ok| {
-        return ok == c1 or ok == c2 or ok == c3;
+        inline for (cs) |c| if (ok | 0x20 == c) return true;
     }
     return false;
 }
@@ -89,12 +75,8 @@ pub fn advance(self: *FloatStream, n: usize) void {
     self.offset += n;
 }
 
-pub fn skipChars(self: *FloatStream, c: u8) void {
-    while (self.firstIs(c)) : (self.advance(1)) {}
-}
-
-pub fn skipChars2(self: *FloatStream, c1: u8, c2: u8) void {
-    while (self.firstIs2(c1, c2)) : (self.advance(1)) {}
+pub fn skipChars(self: *FloatStream, comptime cs: []const u8) void {
+    while (self.firstIs(cs)) : (self.advance(1)) {}
 }
 
 pub fn readU64Unchecked(self: FloatStream) u64 {
diff --git a/lib/std/fmt/parse_float/decimal.zig b/lib/std/fmt/parse_float/decimal.zig
index 4e8a84bcf6be..0b27adabf263 100644
--- a/lib/std/fmt/parse_float/decimal.zig
+++ b/lib/std/fmt/parse_float/decimal.zig
@@ -241,18 +241,18 @@ pub fn Decimal(comptime T: type) type {
             var d = Self.new();
             var stream = FloatStream.init(s);
 
-            stream.skipChars2('0', '_');
+            stream.skipChars("0_");
             while (stream.scanDigit(10)) |digit| {
                 d.tryAddDigit(digit);
             }
 
-            if (stream.firstIs('.')) {
+            if (stream.firstIs(".")) {
                 stream.advance(1);
                 const marker = stream.offsetTrue();
 
                 // Skip leading zeroes
                 if (d.num_digits == 0) {
-                    stream.skipChars('0');
+                    stream.skipChars("0");
                 }
 
                 while (stream.hasLen(8) and d.num_digits + 8 < max_digits) {
@@ -292,13 +292,13 @@ pub fn Decimal(comptime T: type) type {
                     d.num_digits = max_digits;
                 }
             }
-            if (stream.firstIsLower('e')) {
+            if (stream.firstIsLower("e")) {
                 stream.advance(1);
                 var neg_exp = false;
-                if (stream.firstIs('-')) {
+                if (stream.firstIs("-")) {
                     neg_exp = true;
                     stream.advance(1);
-                } else if (stream.firstIs('+')) {
+                } else if (stream.firstIs("+")) {
                     stream.advance(1);
                 }
                 var exp_num: i32 = 0;
diff --git a/lib/std/fmt/parse_float/parse.zig b/lib/std/fmt/parse_float/parse.zig
index 841119aa0207..78af642cdb04 100644
--- a/lib/std/fmt/parse_float/parse.zig
+++ b/lib/std/fmt/parse_float/parse.zig
@@ -100,6 +100,7 @@ const ParseInfo = struct {
 };
 
 fn parsePartialNumberBase(comptime T: type, stream: *FloatStream, negative: bool, n: *usize, comptime info: ParseInfo) ?Number(T) {
+    std.debug.assert(info.base == 10 or info.base == 16);
     const MantissaT = common.mantissaType(T);
 
     // parse initial digits before dot
@@ -107,12 +108,10 @@ fn parsePartialNumberBase(comptime T: type, stream: *FloatStream, negative: bool
     tryParseDigits(MantissaT, stream, &mantissa, info.base);
     const int_end = stream.offsetTrue();
     var n_digits = @as(isize, @intCast(stream.offsetTrue()));
-    // the base being 16 implies a 0x prefix, which shouldn't be included in the digit count
-    if (info.base == 16) n_digits -= 2;
 
     // handle dot with the following digits
     var exponent: i64 = 0;
-    if (stream.firstIs('.')) {
+    if (stream.firstIs(".")) {
         stream.advance(1);
         const marker = stream.offsetTrue();
         tryParseDigits(MantissaT, stream, &mantissa, info.base);
@@ -132,14 +131,14 @@ fn parsePartialNumberBase(comptime T: type, stream: *FloatStream, negative: bool
 
     // handle scientific format
     var exp_number: i64 = 0;
-    if (stream.firstIsLower(info.exp_char_lower)) {
+    if (stream.firstIsLower(&.{info.exp_char_lower})) {
         stream.advance(1);
         exp_number = parseScientific(stream) orelse return null;
         exponent += exp_number;
     }
 
     const len = stream.offset; // length must be complete parsed length
-    n.* = len;
+    n.* += len;
 
     if (stream.underscore_count > 0 and !validUnderscores(stream.slice, info.base)) {
         return null;
@@ -159,7 +158,7 @@ fn parsePartialNumberBase(comptime T: type, stream: *FloatStream, negative: bool
     n_digits -= info.max_mantissa_digits;
     var many_digits = false;
     stream.reset(); // re-parse from beginning
-    while (stream.firstIs3('0', '.', '_')) {
+    while (stream.firstIs("0._")) {
         // '0' = '.' + 2
         const next = stream.firstUnchecked();
         if (next != '_') {
@@ -193,6 +192,9 @@ fn parsePartialNumberBase(comptime T: type, stream: *FloatStream, negative: bool
                 break :blk @as(i64, @intCast(marker)) - @as(i64, @intCast(stream.offsetTrue()));
             }
         };
+        if (info.base == 16) {
+            exponent *= 4;
+        }
         // add back the explicit part
         exponent += exp_number;
     }
@@ -212,17 +214,19 @@ fn parsePartialNumberBase(comptime T: type, stream: *FloatStream, negative: bool
 /// significant digits and the decimal exponent.
 fn parsePartialNumber(comptime T: type, s: []const u8, negative: bool, n: *usize) ?Number(T) {
     std.debug.assert(s.len != 0);
-    var stream = FloatStream.init(s);
     const MantissaT = common.mantissaType(T);
+    n.* = 0;
 
-    if (stream.hasLen(2) and stream.atUnchecked(0) == '0' and std.ascii.toLower(stream.atUnchecked(1)) == 'x') {
-        stream.advance(2);
+    if (s.len >= 2 and s[0] == '0' and std.ascii.toLower(s[1]) == 'x') {
+        var stream = FloatStream.init(s[2..]);
+        n.* += 2;
         return parsePartialNumberBase(T, &stream, negative, n, .{
             .base = 16,
             .max_mantissa_digits = if (MantissaT == u64) 16 else 32,
             .exp_char_lower = 'p',
         });
     } else {
+        var stream = FloatStream.init(s);
         return parsePartialNumberBase(T, &stream, negative, n, .{
             .base = 10,
             .max_mantissa_digits = if (MantissaT == u64) 19 else 38,