Your tax code is one of those small pieces of admin that can quietly cost you hundreds of pounds a year. It sits on your payslip, often ignored, yet it tells your employer exactly how much tax-free income you are entitled to. Get it right and your tax is spot on. Get it wrong and you could be overpaying, underpaying, or facing a surprise bill from HMRC later. The good news? Checking it takes ten minutes and is far less painful than it sounds.

What your tax code actually means

Most tax codes are a number followed by a letter. The number represents your tax-free allowance for the year, divided by ten. So the standard code 1257L means you can earn £12,570 before paying income tax. The letter tells your employer how to apply that allowance. L is the standard code for most people. BR means all income from that job is taxed at the basic rate of 20%, which is common for second jobs. D0 means everything is taxed at 40%, and 0T means you get no personal allowance at all. Emergency codes ending in W1 or M1 tax you as if every payday is your first, which can lead to overpaying if your income varies. If you live in Scotland, your code starts with S; in Wales, it starts with C.

Why your code might be wrong

Tax codes change when HMRC thinks your circumstances have changed. Sometimes they are right, sometimes they are working from outdated information. Common triggers include starting a new job without handing over a P45, taking on a second job, receiving company benefits like a car or private medical insurance, having untaxed savings interest above your personal savings allowance, rental income, or the State Pension. HMRC may also estimate your income incorrectly. If you have recently married, changed jobs, or started receiving a benefit in kind, your code deserves a closer look.

Checking your code against your allowance, benefits and second job

Start by finding your tax code on your payslip or in your HMRC personal tax account. Then work out what your code should be. Add up your tax-free allowances: the standard personal allowance (£12,570 for most people), plus any Blind Person's Allowance or Marriage Allowance you are eligible for. Now subtract anything that reduces your allowance, such as company benefits (a company car, fuel card, or medical cover) and untaxed income like rental profit or the State Pension. The result, divided by ten, should match the number in your tax code. If you have two jobs, only one should carry your main allowance. The other should usually be BR. Two jobs both on 1257L will mean you underpay tax and face a bill.

• One job, no benefits: 1257L is usually correct.
• Second job: the second employer should use BR, not 1257L.
• Company car or medical insurance: your code number will be lower than 1257.
• State Pension or rental income: HMRC may reduce your allowance to collect the tax due.
• Emergency code: W1 or M1 often means you need to check and update your records.

The cost of a wrong code – overpaying and underpaying

If your code is too high, you pay too little tax during the year and HMRC will usually send you a P800 letter asking for the difference. That can be a nasty shock if you have spent the money. If your code is too low, you pay too much and are effectively giving HMRC an interest-free loan. You can claim a refund for overpaid tax going back four years. Underpayments are typically collected by adjusting your tax code for the following year, though you can ask to pay in instalments if it causes hardship. Either way, it is your money and your responsibility to check.

How to fix it – practical steps

Log into your HMRC personal tax account online or through the HMRC app. Go to the PAYE section and check your current tax code, estimated income, and employment details. If something looks wrong, update it online or call HMRC. Have your payslips and P60 handy. If you have started a new job, give your new employer your P45 so they can use the right code. If you have left a job, check your final payslip to make sure the correct code was used. If you have overpaid, request a refund. If you have underpaid, ask about a payment plan. Keep copies of everything you send or receive.

Staying on top of it through life changes

Your tax code is not a one-off job. Review it at the start of every tax year in April, and again whenever your circumstances change: a new job, a second job, a pay rise, a company benefit, marriage, divorce, retirement, or starting to rent out a property. Check your first payslip of the new tax year to confirm the code is correct. Small, regular checks stop small errors becoming large bills. You do not need to be a tax expert. You just need to know what your code should look like and act when it does not match.