TaxSums

How we calculate

Tax calculators are easy to build and easy to get subtly wrong. This page sets out exactly what ours do, so you can judge whether to trust them.

Where the rates come from

Every figure is taken from a primary source: HMRC, Revenue Scotland, the Welsh Revenue Authority, or the Scottish Government. We do not take rates from other calculators, news articles or accountancy blogs, because those introduce a second chance to be wrong.

All rates live in one versioned data file per tax year, typed against a schema that will not compile if a figure is missing. Every entry carries the source URL and the date it was last checked, and both are shown on the pages that use it. See every rate with its source.

How each deduction is worked out

Income tax

Applied progressively to income after the personal allowance. The allowance is reduced by £1 for every £2 of adjusted net income above £100,000. Band limits are measured on taxable income, which is the correct legal basis — a detail that catches out calculators using gross-income equivalents at the additional rate boundary.

National Insurance

Charged on earnings, not taxable income, so the personal allowance is irrelevant and benefits in kind are excluded. We calculate annually. Real payroll assesses each pay period separately and does not true up across the year, so uneven pay produces different payslip figures — every relevant page says so.

Student loans

A percentage of earnings above the plan threshold, rounded down to whole pounds as HMRC does. Postgraduate loans are calculated alongside undergraduate plans, not instead of them. Only salary sacrifice reduces the earnings figure used.

Pensions

We model the three arrangements separately, because they produce different take-home pay for the same contribution:

  • Salary sacrifice reduces the base for income tax, National Insurance and student loans.
  • Net pay reduces the base for income tax only.
  • Relief at source reduces neither, but extends the basic rate band and reduces adjusted net income.

All three reduce adjusted net income, which is why any of them can rescue a tapered personal allowance.

Property taxes

The three UK regimes differ in structure, not just in numbers, so each is modelled on its own terms: England and Northern Ireland add a surcharge to every band, Scotland charges a flat supplement on the whole price alongside the banded tax, and Wales uses an entirely separate band table.

What we test

The calculation engine is pure functions with no interface code, covered by 112 unit tests. Expected values come from HMRC worked examples and published figures, not from our own output. The suite includes the cases most likely to be wrong:

  • The personal allowance taper at £100,000 and £125,140, and the 60% marginal rate between.
  • The National Insurance step at the upper earnings limit.
  • The additional rate boundary at £125,140 of taxable income — where a £150,000 salary must produce exactly £53,703 of income tax.
  • The first-time buyer stamp duty cliff at £500,001.
  • The 26.5% effective marginal corporation tax rate between £50,000 and £250,000.
  • Stacked Plan 2 and postgraduate loan repayments.
  • All six Scottish bands, and the point where Scottish tax overtakes the rest of the UK.

We also cross-check headline results against established calculators. Where we disagree we investigate rather than assume we are right — that is how the additional rate boundary error was found and fixed before launch.

What we do not do

  • We do not collect your data. Every calculation runs in your browser. Nothing you type is sent anywhere or stored.
  • We do not require an account. No sign-up, no email, no paywall.
  • We do not model every edge case. Each tool lists what it cannot handle rather than failing silently.
  • We do not give advice. These are estimates to help you understand your position and ask better questions.

Rounding

Calculations run at full precision and round to the penny at the end. Student loan repayments are the exception — HMRC rounds each down to whole pounds and so do we. Headline figures are displayed to the nearest pound; detailed tables show pence where it matters. You may see a penny of difference against payroll software, which rounds at different points.

Corrections

If you find a figure you believe is wrong, tell us. Send the inputs you used and what you expected. Corrections are the most useful thing anyone sends us and they are fixed the same day, then recorded on the changelog.

These calculators provide information, not financial or tax advice. They cannot account for every personal circumstance. Check anything important with an accountant or with HMRC before acting on it.