How These Numbers Are Worked Out
Exactly what this site computes, how, and where it is uncertain.
UK income tax
Bands are applied to taxable income, which is gross pay less the Personal Allowance actually due after any taper. The additional and top rate boundary sits at £125,140 because the allowance is fully withdrawn by that point, so taxable income and total income coincide there.
Scottish rates apply to non-savings, non-dividend income only. Savings interest and dividends are taxed at UK rates and in UK bands for Scottish taxpayers too, and this site does not model investment income.
The Personal Allowance taper is applied where the tax code is the standard code or none was supplied. If you enter a specific code we honour it exactly, on the basis that anyone genuinely caught by the taper would have been issued a reduced code that already reflects it.
National Insurance
Calculated on the statutory period thresholds — £1,048.00 a month rather than a twelfth of £12,570 — because that is what real payroll does. On a £150,000 salary this moves the answer by about 60p a year compared with an annual calculation, which is why our figure reconciles with a payslip and an annual-threshold calculation does not quite.
Category letters A, B, C, H, J, M, V and X are supported. H, M and V cost the employee exactly the same as A — those letters relieve the employer only, which is a common misconception.
Student loans
Worked out on National Insurance-able earnings in each pay period, with the excess over the period threshold rounded down to whole pounds before the rate is applied, and the deduction rounded down again. That is what makes annual totals slightly lumpy and it is what the rules require.
Plan 5 returns nothing for 2024/25 and 2025/26 because it did not exist in payroll before 6 April 2026. This is deliberate.
Pensions
Three methods are modelled and they behave differently. A net pay arrangement reduces the income tax base only. Relief at source reduces neither base at payroll and gives basic rate relief through the provider. Salary sacrifice reduces gross pay itself, so it cuts income tax, National Insurance and the earnings figure student loans are based on.
The National Insurance cap on sacrificed pension contributions above £2,000, announced at the Autumn Budget 2025, takes effect on 6 April 2029 and is therefore not applied to any year modelled here.
US federal withholding
Publication 15-T Worksheet 1A, implemented in full: annualise, apply the Step 4(a) and 4(b) adjustments, subtract the line 1g offset ($12,900 married filing jointly, $8,600 otherwise, and zero if the Step 2 box is checked), look up the Standard or Step 2 Checkbox schedule, subtract Step 3 credits, and divide back down by the pay periods.
Every row of every table for 2024, 2025 and 2026 is reconciled against the statutory brackets by an automated test before the site is built. The construction rule is that a standard row boundary equals the statutory bracket plus the standard deduction less line 1g, and a checkbox row boundary equals half the sum of the bracket and the standard deduction — with the IRS's own rounding, which rounds boundaries to whole dollars but computes the tentative amounts from the unrounded halves.
The pre-tax rule
A traditional 401(k) deferral reduces federal taxable wages but not Social Security and Medicare wages. A Section 125 benefit reduces both. A Roth 401(k) reduces neither. Three states then diverge from federal treatment: Pennsylvania taxes 401(k) deferrals going in, and California and New Jersey do not recognise HSA contributions. All four rules are implemented and asserted in the test suite.
Where we are uncertain
The statutory School Teachers' Pay and Conditions Document 2026 has not been published; the scales here come from NEU and NASUWT, which agree with each other. Seventeen US states have not fully reissued 2026 withholding rules and each of those pages says so. Scotland, Wales and Northern Ireland's Agenda for Change scales are not reproduced because we could not confirm them from a primary source, and Northern Ireland had not announced an award at all. There is no national NHS on-call table to reproduce — Annex 29 sets principles for local agreements, so any calculator showing a national percentage is showing one trust's scheme.
The sources page lists every rate and where it came from.
What this is not
Guidance, not advice. We are not accountants, tax advisers or financial advisers, and nothing here is a recommendation about your circumstances. For anything consequential, check with someone who is regulated to advise you.