How we calculate

The rules file

All rates and thresholds for 2026/27 live in one file. Each figure carries the official page it came from, a status, and the date it was last checked. The status is one of three: verified (checked against the source on the date shown), announced but not in force (published for a later date and not applied), or assumed (carried forward from the previous year and not yet checked).

Right now all 34 figures are verified, most recently on 20 October 2026. If any figure is ever unverified, every page says so in a banner at the top.

What the engine does

It works in whole pence and rounds once, at display. Income tax is worked out for the year: pay after salary sacrifice, plus taxable benefits and any K-code addition, less the tax-free pay from your tax code after the taper above £100,000 of adjusted net income, through the bands for your region. National Insurance and student loan deductions are worked out on each payday, as payroll does, using HMRC's monthly or weekly thresholds, so a bonus month is right and the year is the sum of the paydays.

We follow PAYE's conventions so the figures match a payslip: tax-free pay is the code number times ten plus nine (1257L is £12,579), a K code adds the same way, taxable pay is rounded down to whole pounds, and each payday's student loan deduction is rounded down to the pound. The band edges are in taxable income, so the higher rate runs to £125,140 of taxable income whatever your allowance. These were checked against a real 2026/27 payslip on a K code with a Plan 2 loan: NI and the loan deduction match to the penny and tax within pence.

What a tax code tells us

The digits give your allowance, an S or C prefix sets your region, M or N shows Marriage Allowance, K means deductions exceed your allowance, BR and D codes mean a flat rate on all pay, and W1, M1 or X mean an emergency code. We decode it under the field and say what we changed. A code cannot tell us why it is reduced, so if yours is below the standard we ask whether that is for benefits, to avoid counting them twice.

What we leave out, for now

Directors' annual NI basis. Mid-year pay changes and the cumulative PAYE catch-up they cause. More than one job beyond the hint a BR code gives. The 50% per-payday cap on K codes. Higher-rate relief on relief-at-source pensions and Gift Aid is shown as if claimed from HMRC, which you have to do yourself. Each is noted on the page where it would matter.

Not advice

These are estimates from published rules. They are not personalised financial advice and they are not a substitute for your payslip, your P60 or HMRC. If a decision turns on a figure here, check it with HMRC or a regulated adviser first.

Sources