Tax Code Checker United Kingdom

What the tax code checker tells you

Your tax code is the only instruction HMRC gives your employer, and if it is wrong nothing else on the payslip makes up for it. Enter the code, your salary and any adjustments, and the tool shows the tax-free pay the code gives you, the tax it will collect over a full year, the tax actually due and the difference between them.

A positive difference means the code is taking too much and a refund is building up. A negative one means too little is being collected and a bill is building up, which is better found in May than in January.

How a code turns into tax-free pay

Most codes are a number followed by a letter. The number is your free pay for the year without its final digit, so 1257L gives £12,570 and 1000L gives £10,000. The letter tells the employer which rules apply but does not change the arithmetic: L is the standard allowance, M and N show that marriage allowance has been received or transferred, and T means other adjustments are included.

Some codes are letters only. BR taxes every pound at the basic rate. D0 taxes everything at the higher rate and D1 at the additional rate, common on a second income for someone whose main job already uses the higher bands. 0T gives no allowance but applies the normal bands in order, so it matches BR on lower pay and takes more once pay passes the basic rate band. NT means no tax is deducted, which is rare and usually temporary.

A K code works the other way. When the amounts HMRC wants to tax through your salary exceed your allowances, the balance becomes a negative allowance, and the employer adds the K amount to your taxable pay: a K300 code adds £3,000 to the salary before the bands are applied.

Why your code drifts away from the right number

The allowance in a code is a forecast based on last year’s information, and it goes out of date when your circumstances change. Four things move it most.

Benefits in kind are the most common. A company car, fuel, medical insurance or a cheap loan is taxable income that never appears in your pay, so HMRC collects the tax by reducing your code. Swapping a car mid-year for a higher-emission model can move the code by thousands.

Untaxed income comes next. Modest rental profit, freelance income or savings interest above your Personal Savings Allowance can be collected through your code instead of a tax return, which also reduces the code number.

Allowable expenses push the other way. Professional subscriptions, union fees and tools increase your allowance and raise the code number, and they are often missed.

The personal allowance itself is not fixed. Above £100,000, £1 of allowance is withdrawn for every £2 of income, and by £125,140 none is left. A code that still shows the full allowance on a six-figure salary is one of the most expensive PAYE errors, because the shortfall builds at the higher rate all year.

Reading the results

Start with the two allowance figures. The tax-free pay your code gives is what the payslip uses. The tax-free pay you should have is rebuilt from the statutory allowance, its taper and the adjustments you entered. If they match, the code is right.

If they differ, the suggested code shows what the corrected code would look like: the allowance without its last digit, or a K code if the result is negative. The per-period figures show how much each payslip is out by, which is usually the most useful number when you contact HMRC. The bar chart puts the two annual tax figures side by side.

Second jobs, pensions and split allowances

One allowance has to cover every source of PAYE income, and HMRC decides where to put it. Usually the whole allowance goes to the main job and the second source is taxed at a flat rate, which is why a part-time job or small pension often comes with a BR or D0 code.

That is not always right. If the main job pays less than the allowance, part of it goes unused while the second job is taxed from the first pound, and the difference is recovered only after the tax year ends. HMRC can split the allowance between the two sources, giving each a smaller code number, and that is worth asking for when your main income is close to or below the allowance.

The State Pension is paid without tax deducted, so HMRC collects the tax on it by reducing the code on a private pension or salary. That is the most common reason a retiree ends up on a K code, so check the adjustment against your actual State Pension.

Run each source through the tool separately, using the code on that payslip, to see the whole picture.

When the code is right but the payslip still looks wrong

A non-cumulative code, marked W1, M1 or X, ignores everything earned earlier in the year and taxes each period on its own, so someone starting work in the autumn does not get the allowance they have built up. The tax settles once the code becomes cumulative or the year ends.

A mid-year change can also look odd. When a code is corrected on a cumulative basis, the next payslip carries the whole year-to-date adjustment at once. Comparing annual figures, as this tool does, sees through both effects.

The figures are an estimate for a full year at the salary you enter, assuming even pay across the year. They are not personal tax advice; use your payslip, P60 or personal tax account to confirm your position before asking for a change.

About This Tax Code Checker

Use this tax code checker to get an instant, accurate estimate. The figures follow official GOV.UK guidance and every rate and threshold is kept up to date for 2026. Adjust the inputs above to match your situation, or browse all our calculators to plan your next step.