How to check your tax code is right

A tax code is normally numbers followed by a letter. Multiply the numbers by ten to get the amount of income HMRC expects you to receive tax-free in the year. The standard code while the personal allowance is £12,570 is 1257L.

The letter modifies it. L is the standard personal allowance. M and N indicate the Marriage Allowance has been transferred to or from you. T means there are other calculations involved. 0T means no personal allowance is being applied at all — usually because HMRC has no information about you. BR means all income from that source is taxed at the basic rate, D0 at the higher rate and D1 at the additional rate; these are normal on a second job or a pension, and wrong on a sole job. K codes mean your untaxed income or benefits exceed your allowance, so an amount is added to your taxable pay rather than deducted.

A prefix matters too. S means you are a Scottish taxpayer and Scottish rates apply; C means Welsh. These are set by where you live, not where you work, and HMRC determines it from your address.

A suffix of W1, M1 or X means an emergency code. It taxes each pay period in isolation rather than cumulatively, so it ignores any unused allowance from earlier in the year. This is the single most common cause of overpayment for people starting a new job.

Sign in to your personal tax account on GOV.UK or open the HMRC app. Both show your current code, the income HMRC thinks you receive from each source, and the calculation behind the code.

Look specifically at: whether HMRC thinks you have a job you no longer have, whether a company benefit such as a car or medical insurance is listed at the right value, whether an estimated income figure is wildly wrong, and whether a state pension or other income is included that should not be.

You can update most of these yourself in the app or online, and HMRC will reissue the code to your employer. This is far faster than phoning.

Check again after any change: starting or leaving a job, taking a second job, starting to receive a pension, getting a company car, or a large change in income.

For the current tax year, correcting the underlying information in your personal tax account usually triggers a reissued code within days. Once your employer applies it, any overpayment made earlier in the same tax year is normally refunded automatically through payroll.

For previous tax years, HMRC reconciles after the year ends and issues a P800 calculation if you have overpaid or underpaid. A P800 showing a refund can usually be claimed online, with the money paid within about five working days, or sent as a cheque if you do nothing.

You can claim back overpaid tax for up to four previous tax years. That is a genuine deadline: the 2021–22 year, for example, closes on 5 April 2026. If you think you have overpaid in an earlier year and no P800 has arrived, contact HMRC directly rather than waiting.

Common causes of refunds beyond wrong codes: work expenses you were entitled to claim, such as professional subscriptions, uniform maintenance or mileage in your own vehicle; the Marriage Allowance where one partner earns below the personal allowance; and being taxed on the emergency code after a first pension withdrawal.

Do not use a refund company unless you have a genuinely complex claim. They charge a substantial percentage of a refund you can claim yourself in minutes, and several have been the subject of regulatory action over misleading practices and assignment clauses that divert future refunds too.

The reverse happens too. If your code has been too generous, a P800 will show an underpayment, and HMRC will normally collect it by adjusting next year's code rather than demanding a lump sum.

If the amount is large, or you no longer have PAYE income, HMRC will ask for direct payment and will usually agree a Time to Pay arrangement if you cannot pay at once. Contact them before the deadline — arrangements are routinely available to people who ask early and considerably harder to obtain after enforcement has begun.

Underpayments caused by HMRC's own error can sometimes be written off under Extra-Statutory Concession A19, where HMRC failed to act on information it held in good time and you reasonably believed your affairs were in order. It is applied narrowly, but it is worth citing where the facts fit.