How our calculators work

This page explains the method behind each calculator, the choices we made where the rules leave room for them, and what the calculators do not cover.

Take-home pay

The take-home pay calculator works out a full year at once, in this order.

Any salary sacrifice comes off your salary first. What is left is the pay used for National Insurance and student loans.
A net pay pension also comes off before income tax, but not before National Insurance.
Your personal allowance is £12,570. If your adjusted net income is over £100,000, the allowance falls by £1 for every £2 above that figure, in whole pounds. Payroll then applies the allowance through your tax code: the code is the allowance divided by ten, and payroll adds £9 to the code times ten. For the standard code, 1257L, that gives £12,579 of tax-free pay a year. The calculators use the same rule, so they match HMRC's own take-home estimator.
Income tax is charged on the rest of your pay, band by band, at the rates for England, Wales and Northern Ireland or for Scotland.
With a relief at source pension in England, Wales or Northern Ireland, your basic rate band is made longer by the gross contribution, which is how HMRC gives the extra relief to higher rate taxpayers.
Employee National Insurance is charged on your yearly pay using the yearly thresholds.
Student loan repayments are charged on your yearly pay above each plan's yearly threshold and rounded down to the whole pound, as payroll software does.
Take-home pay is your salary minus income tax, National Insurance, student loan repayments and the pension you pay from your salary.

Rounding

Each amount is rounded to the nearest penny, with halves rounded up, before it is added to anything else. Because of this, the lines of the working always add up to the total shown. Monthly, weekly, daily and hourly figures are the yearly take-home pay divided by 12, 52, 260 and 52 times your weekly hours.

Why your payslip can differ

Employers work out tax and National Insurance separately for each pay period, using the thresholds for a week or a month. Those thresholds are the yearly figures divided and then rounded, so a payslip can differ from our monthly figure by a few pence. Over a full year on the same salary, the totals come out very close.

The tax-free pay from your tax code is £9 more than the personal allowance, so payroll takes slightly less income tax than the law strictly charges over a year. The difference is £1.80 a year at the basic rate, and the figures on this site follow payroll.

Your payslip will also differ if your tax code is not the standard one, if you have benefits in kind such as a company car, if your pay changes during the year, or if you started the job part way through the year.

What the take-home calculator leaves out

Tax on savings, dividends, rent or self-employed income.
The Marriage Allowance, the Blind Person's Allowance and other changes to your tax code.
The extra pension tax relief that Scottish taxpayers above the basic rate claim through Self Assessment. The calculator says so when this applies.
The High Income Child Benefit Charge.

How we test the sums

Every calculator is written twice, by two separate programs that do not share any code. Both read the same list of official rates. A test runs both programs over hundreds of cases, including the salaries where a new band or threshold starts, and fails if any figure differs by a penny or more. The first time we ran this test, it found a rounding error in half-penny cases, which we fixed before the site went live.