How to check your tax code and the allowances in it

Every UK resident has a tax code, assigned by HM Revenue & Customs (HMRC), and that includes people who do not pay tax at all¹. The code tells your employer or pension provider how much tax-free income you are entitled to before tax is taken from your pay. Checking it takes a few minutes, and it is worth doing because a wrong code means the wrong amount of tax comes out of your wages.

You can check your tax code online on GOV.UK or in the HMRC app, and let HMRC know if it still does not look right². The app lets you check your tax code, National Insurance number, employment history and much more³. If you would rather speak to someone, HMRC's tax codes helpline is 0300 200 3300⁴.

HMRC sends details of your tax code before 6 April each year, ahead of the start of the new tax year⁵. Codes change for ordinary reasons: a new job, a pay rise, a company car, extra pension income, savings interest or a change to work benefits². In most cases, if your tax code changes, there is nothing for you to do².

Every UK resident has a tax code from HMRC

All UK residents are each assigned a tax code by HMRC, even if you are a non-tax payer¹. Income Tax in the UK is administered by HMRC, which also runs the personal tax credits, child benefit and tax-free childcare systems across the UK⁸. The Survey of Personal Incomes, the official statistics on income tax, is based on information held by HMRC on individuals who could be liable to UK Income Tax¹⁰.

A tax code is not a bill and it is not a statement of what you owe. It is an instruction to whoever pays you, telling them how much of your income to leave untaxed before they apply tax to the rest. That is why it matters even to people whose income is below the point at which tax starts: the code is how the system knows to leave your pay alone.

If you do not pay income tax because you are on a low income, you automatically get tax relief on your workplace pension contributions¹¹. That relief is delivered through the pension arrangement rather than through your tax code, but it is one of the reasons a code can look different from what you expect.

A code beginning with the letter K works differently from most. Tax codes with a K mean you have income or deductions which are higher than your tax-free Personal Allowance and are not already being taxed¹². In practice that means more tax is taken from your pay, not less, because the code is collecting tax on income that has no allowance left to cover it.

Check your tax code online or in the HMRC app

The quickest route is HMRC's own services. You can check your tax code online on GOV.UK or on the HMRC app². You can use the PAYE section of HMRC online services or the HMRC app to check employers, estimated income and tax codes¹³. If you have a Personal Tax Account, you can use the HMRC app to access your account¹⁴.

The app does more than show a code. It allows customers to check their pay before pay day and check their National Insurance number, tax code, income and benefits, and users can make Self Assessment payments, track letters, get tax estimates and more¹⁵. Customers can also view their tax code, PAYE details, employment history and Self Assessment record¹⁶. You can also use the HMRC app to check how much Income Tax you paid last year¹⁷.

If you cannot create a personal tax account, contact HMRC for advice; you can also estimate your Income Tax for previous years without signing in¹⁷. Your National Insurance number, which sits alongside your tax code in most of these services, can be checked using your online Personal Tax Account or on the HMRC app¹⁸.

Your code also appears on paper and in messages. It is on letters or emails from HMRC, and on your payslips from April 2025⁶. The emergency tax code will be written on your payslip, and you can generally find it near your National Insurance number⁷. HMRC will send your information about what tax code they are using¹⁹, and you can choose to receive updates about any changes to your tax code by email or as an online message in your HMRC account or HMRC app¹³.

Why your tax code can change at the start of the tax year

The tax year runs from 6 April to 5 April, and HMRC reviews codes at the start of it. HMRC adds an estimated amount in your tax code for the current tax year based on the information given to them by your bank or building society for the previous tax year²⁰. You can see the estimated amount in the Personal Tax Account²⁰. That single mechanism explains a great many April surprises: interest that was paid to you a year ago can change what comes out of your pay now.

Other triggers are more personal. When something happens in your life like you start a new job, get a pay rise or a company car, it can affect how much tax you pay². HMRC may update your tax code part way through the year if they are informed of a change in your circumstances⁶. The list of triggers includes starting a new job, a pay rise or a company car, extra income from workplace or private pensions, interest on savings, or changes to work benefits like company healthcare².

A K code has its own set of causes. It can happen when you are paying tax you owe from a previous year through your wages or pension, getting State Pension or taxable state benefits, getting company benefits you need to pay tax on such as a company car, or getting more interest on your savings than your Personal Savings Allowance¹².

If a P800 tax calculation letter shows you owe tax, HMRC will usually change your tax code for the following year so it can collect the money that way²¹. When your P2 Tax Coding Notice arrives, check that your income is correct, that any taxable company benefits you get are listed, and the tax year at the top of the letter, and let your employer know²¹.

Only HMRC issues tax codes: what your employer can and cannot do

Your tax code is issued by HMRC⁶. Your employer applies it through the payroll, but cannot set it, change it or decide what allowance you get. If you have more than one job you will have a tax code for each employment¹³, and HMRC decides how your allowance is divided between them.

In most cases, if your tax code changes, there is nothing for you to do². The employer tells HMRC and the PAYE system works out the new tax. There is one timing point worth knowing: speak to your employer if the change to your tax code has not been applied by your next pay day, if you are paid monthly, or your third pay day, if you are paid weekly³.

When you start a job, the paperwork you hand over helps HMRC get the code right. That means giving your employer your P45; if you do not have a P45, your employer will ask you to fill in a starter checklist⁷. A P45 shows how much you have earned, how much tax you have paid that year since 6 April, and what tax code your employer has been using¹⁹.

HMRC can also ask for information, but only within limits. At the end of each tax year, HMRC checks how much tax you have paid using information from employers, pension providers, banks and building societies²².

When your tax code looks wrong: tell HMRC

If you think the tax is wrong, your tax code may be incorrect²³. If HMRC do not have accurate information about your income, you may be issued with an incorrect tax code⁶. It is very important to keep HMRC informed of changes that may impact your tax position so that they can adjust your tax code⁶.

The first step is to contact HMRC to update your tax code so they can adjust it⁶. You can use HMRC's online Income Tax checker, or call 0300 200 3300⁷. Check with HM Revenue & Customs that you have the right tax code²⁴. If you are a higher rate taxpayer you can also contact HMRC¹¹.

There is a formal route as well as an informal one. An individual may object to a tax code adjustment by contacting HMRC or by making a formal objection under regulation 18 of the PAYE Regulations²⁵. If your tax return contains an obvious error or is missing information, HMRC corrects the return and sends a notice explaining why²⁶.

Emergency codes are common at the start of a job. In your first job, or first day in a new job, you may be given an emergency tax code³. If your tax code changes to have a W1, M1 or X at the end, you are on what is called an emergency tax code². It usually means HMRC is waiting for some more leaving details from your old employer, or that you have not filled in the new starter checklist². It is worth checking if you have been at your new job for more than three months and you are still paying emergency tax⁷.

If HMRC cannot collect what you owe through your code, it uses a different route. HMRC sends you a Simple Assessment tax bill, also known as a PA302, if you did not pay enough tax and they could not collect it through your tax code²². If you do not have a tax code or it cannot be changed, HMRC may send a Simple Assessment letter²⁰. Where possible, HMRC will update your tax code, which will determine how much Self Assessment tax is collected through your PAYE income alongside your existing tax on your employment or pension²⁷.

You can also claim back overpaid tax. You can check and claim for a refund online on GOV.UK, or on the app, or by asking HMRC to send you a cheque². If you are an employee, you will not usually have to deal with HMRC unless your tax code is wrong or HMRC has contacted you, you are claiming expenses for professional subscriptions, tools or working from home, you have several jobs or pensions or untaxed income, or you are self-employed as well as employed²⁸.

Where to get free help

If a tax problem is causing you financial difficulty, free and impartial help exists. Contact HMRC to check²⁹, and if you are dealing with debt, StepChange offers debt information²⁹. For benefits questions, StepChange also has further support on government benefits³⁰. The HMRC Tax Credit Enquiries line is 0345 300 3900³⁰.