How we calculate

One engine, one rates file, one test suite. Every calculator on this site uses the same code and the same data, so a figure can never disagree with itself from one page to another.

The calculation order

1. Start with contractual gross pay. Any salary sacrifice reduces the gross first — tax, National Insurance and student loan are then worked out on the reduced figure.
2. Pension. The default is the net-pay model: your contribution comes off gross pay before income tax, but not before National Insurance. Auto-enrolment percentages apply to qualifying earnings (£6,240–£50,270), not to your full salary, unless you choose "% of full salary".
3. Income tax. Taxable pay is gross minus pension minus your personal allowance (£12,570 on tax code 1257L). The allowance tapers by £1 for every £2 above £100,000 and reaches £0 at £125,140. England, Wales and Northern Ireland use the 20% / 40% / 45% bands; Scotland uses its six bands (19% to 48%). Flat codes (BR, D0, D1, 0T, NT) and K codes override the band path as HMRC defines them.
4. National Insurance. Charged per pay period on gross pay before pension: 8% between £12,570 and £50,270 a year, then 2% above. Monthly and weekly views use the per-period thresholds (£1,048 / £4,189 a month; £242 / £967 a week).
5. Student loan. Charged above your plan's threshold — 9% for Plans 1, 2, 4 and 5, 6% for Postgraduate. A Postgraduate loan stacks with an undergraduate plan; each is charged above its own threshold.
6. Take-home pay is gross minus pension, income tax, National Insurance and student loan.

Rounding

Full precision is kept internally. Each displayed money line is rounded to the nearest penny, half-up. Annual figures are computed from annual inputs, not by multiplying rounded monthly figures — so a monthly figure times twelve can differ from the annual figure by a penny. That is expected, not an error.

Rates as data

Every threshold and rate lives in one versioned file (public/data/rates-2026-27.json). No rate is hard-coded inside a calculator. A new tax year means a new file, a full test run, and a changelog entry — never a quiet edit to a page.

The test suite

An automated test suite runs on every build. It includes the three acceptance tests from the specification, reproduced to the penny:

• £35,000 with auto-enrolment pension at 5% of qualifying earnings: pension £1,438.00, tax £4,198.40, NI £1,794.40, net £27,569.20 a year (£2,297.43 a month).
• £110,000 with a Plan 2 loan: personal allowance £7,570, tax £33,432.00, NI £4,210.60, loan £7,255.35, net £65,102.05 a year — marginal rate 71%.
• £50,000 in Scotland: tax £8,982.05, NI £2,994.40, net £38,023.55 a year — £1,496.05 more tax than in England.

Boundary cases (£12,570, £50,270, £100,000, £125,140), every tax code, every pension mode, student-loan stacking and the per-period NI thresholds are tested too. The suite must pass 100% before launch and after every rates change. No launch ever ships with a known-wrong figure.

What the calculators do not do

They do not give financial advice, determine IR35 status, or replace HMRC. They model the standard PAYE position for a single employment; unusual cases (benefits in kind beyond the basic field, multiple jobs, redundancy, maternity and sick pay) have their own tools or are labelled as estimates where the rules are genuinely uncertain. Try the method yourself in the take-home pay calculator — every figure it shows comes from the 2026/27 rates on this page's sister page.