Every number in the calculator, back to a primary source.

The statutory basis for UK income tax rates and bands, and for the withdrawal of the Personal Allowance above £100,000 of Adjusted Net Income.

1.1

Income Tax rates and Personal Allowances

GOV.UK · HMRC · Updated each tax year

https://www.gov.uk/income-tax-rates

Publishes the headline Personal Allowance (£12,570), basic-rate limit (£37,700), higher-rate threshold (£50,270), and additional-rate threshold (£125,140) used in §§02–03 of the methodology.

1.2

Income Tax rates and allowances for current and previous tax years

GOV.UK · HMRC · Historic reference

https://www.gov.uk/government/publications/rates-and-allowances-income-tax/income-tax-rates-and-allowances-current-and-past

Used to verify that the headline thresholds remain frozen at their 2021/22 nominal values through 2027/28, which is the mechanism driving “fiscal drag” as referenced in methodology §03.

1.3

Income Tax Act 2007, section 35 — Personal allowance

legislation.gov.uk · Primary legislation

https://www.legislation.gov.uk/ukpga/2007/3/section/35

Statutory definition of the Personal Allowance and the formula by which it tapers above £100,000 of adjusted net income: £1 reduced for every £2. This is the black-letter source for the 60% marginal-rate arithmetic.

The HMRC definition of Adjusted Net Income drives every income-tested threshold the calculator models.

2.1

Personal Allowances: adjusted net income

GOV.UK · HMRC guidance

https://www.gov.uk/guidance/adjusted-net-income

The canonical definition: total taxable income minus gross pension contributions paid under the relief-at-source method and gross Gift Aid donations. Used throughout methodology §01 to establish the “one number” the calculator pivots on.

2.2

Tax on your private pension contributions — Tax relief

GOV.UK · HMRC guidance

https://www.gov.uk/tax-on-your-private-pension/pension-tax-relief

Distinguishes the three pension-contribution mechanics — relief-at-source, net-pay, and salary-sacrifice. This is the basis for the calculator asking users explicitly which scheme they use, as discussed in methodology §07.

The HICBC band was reset to £60,000–£80,000 from April 2024 (Spring Budget 2024). These sources establish the thresholds and the per-child rates used in methodology §05.

3.1

High Income Child Benefit Charge

GOV.UK · HMRC guidance

https://www.gov.uk/child-benefit-tax-charge

Establishes the £60,000 lower threshold, the 1%-per-£200 clawback, and the £80,000 full-withdrawal point. This is the source for the HICBC formula: CB × min(1, max(0, (ANI − 60000) ÷ 20000)).

3.2

Child Benefit — How much you can get

GOV.UK · HMRC rates

https://www.gov.uk/child-benefit/what-youll-get

Weekly rates used to derive the annual figures in the calculator: £27.05/wk for the eldest or only child, £17.90/wk for each additional child. Annualised at 52 weeks: £1,407 / £2,337 / £3,268 for one, two, and three children respectively.

3.3

Spring Budget 2024 — HICBC threshold change

HM Treasury · Budget document

https://www.gov.uk/government/publications/spring-budget-2024

Announced the raise from the previous £50,000–£60,000 HICBC band to the current £60,000–£80,000 band, effective April 2024 and unchanged into 2026/27. Used for provenance when readers verify that the band referenced is current.

3.4

Claim Child Benefit if you cannot claim online

GOV.UK · HMRC

https://www.gov.uk/child-benefit/how-to-claim

Source for the opt-out NI-credits warning in methodology §05: a non-earning parent can lose State Pension qualifying years if they never register a claim, even when the earning partner’s HICBC would wipe out the benefit.

Class 1 (employee) rates and thresholds, as used in methodology §04. The employee main rate was reduced from 10% to 8% in April 2024 and remains at 8% for 2026/27.

4.1

National Insurance rates and categories

GOV.UK · HMRC

https://www.gov.uk/national-insurance-rates-letters

Publishes the employee main rate (8%), additional rate (2%), the Primary Threshold (£12,570) and the Upper Earnings Limit (£50,270) used in the calculator’s NI formula.

4.2

Rates and thresholds for employers 2026 to 2027

GOV.UK · HMRC technical

https://www.gov.uk/guidance/rates-and-thresholds-for-employers-2026-to-2027

The machine-readable annual reference: all PAYE/NI thresholds used in the calculator in a single table. Cross-checked against §4.1 for the 2026/27 values.

The £100,000 ANI cliff applies per parent — either partner exceeding the threshold is sufficient to disqualify the household. These sources back methodology §06.

5.1

Tax-Free Childcare

GOV.UK · HMRC

https://www.gov.uk/tax-free-childcare

Scheme overview: £8-in, £2-added top-up mechanics with a £2,000 annual cap per child (£4,000 if the child is disabled), and the £100,000 expected-adjusted-net-income eligibility cap.

5.2

Tax-Free Childcare — eligibility

GOV.UK · HMRC eligibility

https://www.gov.uk/tax-free-childcare/if-youre-employed

Confirms that the £100,000 ceiling applies to expected adjusted net income in the current tax year, and that the test is applied to each parent individually — the source for the cliff-edge framing in methodology §06.

Funded childcare rules are devolved. The calculator models the England scheme only, as flagged in methodology §§06 and 09.

6.1

30 hours free childcare

GOV.UK · DfE / HMRC

https://www.gov.uk/30-hours-free-childcare

Confirms the 30 hours-per-week entitlement across 38 term-time weeks, the age range (9 months to school age in 2026/27 after the rollout completes), and the £100,000 ANI cap per parent.

6.2

Spring Budget 2023 — childcare expansion announcement

HM Treasury · Budget document

https://www.gov.uk/government/publications/spring-budget-2023

Announced the phased expansion of 30 funded hours to children aged 9 months to school age, rolled out from April 2024 through September 2025. Used to verify that the full entitlement applies across tax year 2026/27.

The calculator assumes contributions fall within the standard Annual Allowance. These sources underpin methodology §07 and the “71%” headline marginal saving figure.

7.1

Tax on your private pension contributions

GOV.UK · HMRC

https://www.gov.uk/tax-on-your-private-pension

Source for the £60,000 standard Annual Allowance used as the implicit cap on the calculator’s contribution slider. Exceeding this without adequate carry-forward incurs an annual allowance charge, which the calculator does not model.

7.2

Work out your reduced (tapered) annual allowance

GOV.UK · HMRC

https://www.gov.uk/guidance/pension-schemes-work-out-your-tapered-annual-allowance

The tapered annual allowance applies to very high earners (threshold income above £200,000 and adjusted income above £260,000). The calculator explicitly does not model this — users at that level are directed to a chartered adviser in methodology §09.

7.3

Salary sacrifice for employers

GOV.UK · HMRC employer guidance

https://www.gov.uk/guidance/salary-sacrifice-and-the-effects-on-paye

Establishes that salary-sacrifice contributions reduce gross pay before tax and NI are assessed, and therefore save employee NI in addition to income tax — the arithmetic underlying the ~71% marginal-saving headline in methodology §07.

This reference is reviewed after each fiscal event (Spring Budget and Autumn Statement), and again on 6 April at the start of every tax year. If you find an error, please email hello@marginalrate.co.uk with the page anchor and the value you believe is incorrect — corrections are logged and the review date below is advanced.

No part of this calculator is derived from proprietary, gated, or paywalled data. If a source above is unreachable, the Internet Archive holds copies of every GOV.UK page linked here; substitute https://web.archive.org/web/*/ in front of the URL.

8.1

Internet Archive — Wayback Machine

archive.org · Citation fallback

https://web.archive.org/web/2026*/https://www.gov.uk/income-tax-rates

Use the Wayback Machine to retrieve historical snapshots of any GOV.UK page referenced above. Useful for verifying the figures that were current at the time this calculator was last reviewed (20 April 2026).