Why does a salary calculator differ from my payslip?

£3,000 a month on two codes

Both payslips here are £3,000.00 a month, in the same job, with no pension and no student loan. The only difference is the code payroll holds. The figures are estimates for the current tax year.

On 1257L, one month worked on its own has Income Tax of £390.20 and National Insurance of £156.16, so the payslip nets £2,453.64. On level pay a cumulative code like this moves by pence from one month to the next. On 0T M1, which gives no personal allowance and taxes each period by itself, Income Tax is £600.00 on the same National Insurance, so the payslip nets £2,243.84. The gap is £209.80 in that month, about £2,520 over a year if the code is never corrected. A calculator left on 1257L would say the second payslip is about £210 short. The payslip is right for its code, so the code is what to query.

A pension or a loan the calculator was not told about does the same: a 5% relief-at-source pension and a Plan 2 loan on this pay take the net to £2,305.44, and a calculator set to neither reports a shortfall of about £148 made up of those two deductions.

What else moves a payslip away from the estimate

A gap that repeats every month points at the code, the pension method or the loan plan. A gap in one month points at timing.

• The tax code. A calculator assumes 1257L unless you change it. Payroll uses the code HMRC issued, which can carry a K adjustment for benefits or underpayments, a BR or D0 rate for a second job, or a Scottish S prefix.
• The PAYE basis. A cumulative code works tax on everything earned so far this tax year, so a pay rise, a late start or a refund moves this pay period's tax away from a smooth twelfth. A W1, M1, X or NONCUM marker taxes each pay period by itself.
• The pension method. Relief at source takes the contribution from pay after tax and the provider claims the relief, a net pay arrangement takes it before tax, and salary sacrifice gives up pay before tax and National Insurance. The same 5% gives three different net pay figures.
• The student loan plan. Each plan repays above its own threshold, and a postgraduate loan is taken on top of an undergraduate plan.
• Timing. A bonus, overtime or arrears in one period are taxed in that period against the year-to-date position, while a calculator spreads pay evenly.

What the payslip checker does with this

The checker on the payslip diagnostic page takes the payslip's gross pay, pay period, tax code, student loan plan, pension rate and basis, and the deductions it shows. It works out what that pay should have netted on that code and compares each line: Income Tax, National Insurance, pension, student loan and net pay. A cumulative tax line can only be indicative, because a mid-year change moves it. A non-cumulative code, National Insurance and the loan compare directly. It flags an emergency or non-cumulative marker even when the arithmetic is right. It cannot see HMRC's or payroll's records, so it narrows what to ask about and does not decide who is right. Nothing you type leaves your browser.