Tax Codes Explained: How to Tell If You’re Paying Too Much

Your tax code is one of the most important numbers in your working life, and most people never check it. It tells your employer how much income tax to take off your pay. Get it wrong and you can overpay for months without noticing, or occasionally underpay and get a nasty bill later. The good news is that overpaid tax can usually be reclaimed.

This guide decodes what your tax code actually means, shows you the warning signs that yours might be wrong, and explains how to get back anything you have overpaid.

What is a tax code?

A tax code is a short combination of numbers and a letter, for example 1257L. Your employer or pension provider uses it to work out how much of your income is tax free and how much should be taxed. HMRC issues the code, not your employer, so if it is wrong the fix usually starts with HMRC.

What do the numbers and letters mean?

The number generally relates to how much tax free income you are entitled to in the year. Take the standard code 1257L: the 1257 broadly reflects a personal allowance of £12,570, the amount most people can earn before paying income tax. Multiply the number by ten and you get the tax free amount the code is giving you.

The letter describes your situation. Some of the most common are:

• L: you get the standard tax free personal allowance.
• BR: all income from this job or pension is taxed at the basic rate, with no tax free allowance applied here. Common for second jobs.
• D0: all income taxed at the higher rate. Again often a second income.
• 0T: no tax free allowance is applied, often used when HMRC does not have enough information.
• W1, M1 or X: these signal an emergency tax code.

What is emergency tax?

An emergency tax code is a temporary code used when HMRC does not yet have full details of your income, for example when you start a new job, return to work, or start drawing a pension. You will often see W1 (week one) or M1 (month one) after the numbers.

Emergency codes tax each pay period in isolation, ignoring what you have earned and paid so far in the year. The result is that you often pay too much, because the code does not spread your allowances properly. Emergency tax is usually corrected automatically once HMRC catches up, but not always, and not always for the full amount you overpaid. That is when a refund is due.

Signs your tax code might be wrong

It is worth checking if:

• You have recently started a new job or have more than one job.
• You see BR, D0, 0T, or a W1 or M1 code on your payslip and are not sure why.
• Your take home pay dropped without explanation.
• You started or stopped receiving a workplace pension or benefits such as a company car.
• You had a gap in employment during the tax year.
• Your tax code changed midway through the year.

None of these automatically mean you have overpaid, but each is a common trigger for it.

How to check your tax code

Your tax code appears on your payslip, your P60 at the end of the tax year, and any P45 when you leave a job. You can also see it in your HMRC personal tax account online, which shows how HMRC has worked it out and lets you flag anything that looks wrong.

If your code looks off, the underlying issue is often that HMRC has incomplete or outdated information about your income, allowances or expenses, including the very work expenses covered elsewhere on this blog. Claiming those expenses can itself change your code so you pay less going forward.

How to claim back overpaid tax

If you have overpaid through the wrong tax code, you can usually reclaim it. In some cases HMRC issues a P800 calculation at the end of the tax year and refunds you automatically. In others, you need to prompt them, especially where the overpayment is linked to unclaimed expenses or a code that was never corrected.

TaxPro can review your tax codes across recent years alongside any work expenses you are owed, identify where you have overpaid, and put the claim to HMRC for you. Often a tax code problem and an unclaimed expense sit side by side, and dealing with both at once gives a bigger and cleaner result. We work on a no win, no fee basis, so a percentage fee only applies if a refund is secured.

Frequently asked questions

Is emergency tax always wrong?

Not wrong as such, but it often means you pay too much temporarily. It usually corrects itself, but any genuine overpayment can be reclaimed if it is not put right.

My second job is on a BR code. Is that a mistake?

Not necessarily. A BR code on a second job can be correct, because your tax free allowance is usually applied to your main job. The issue is making sure your allowance is in the right place overall.

How far back can I reclaim overpaid tax?

As with employment expenses, claims generally cover the current tax year plus the previous four.

Will checking my tax code trigger an investigation?

No. Checking and correcting your tax code is a normal, routine part of the PAYE system.

Think your tax code is costing you? Check it

A wrong tax code can quietly cost you for months. It is worth finding out whether you have overpaid, and reclaiming it if you have.

Start your claim with TaxPro and we will review your tax codes and expenses together and reclaim what you are owed, on a no win, no fee basis.

TaxPro works on a no win, no fee basis. If we secure your refund, our fee for PAYE claims is 37.5% plus VAT (minimum £50 plus VAT). If there is no refund, there is nothing to pay.

The common UK tax codes explained

Every tax code means something different, and being on the wrong one can quietly cost you money. We have a full guide to each of the most common codes:

• 1257L tax code — the standard code for most people.
• BR tax code — basic rate on all income, common for second jobs.
• 0T tax code — no tax free allowance applied.
• D0 tax code — higher rate (40%) on all income.
• K tax code — deductions exceed your allowance.
• T tax code — items in your code under review.
• Emergency tax code (W1, M1 or X) — temporary, often means you overpay.

Scottish and Welsh tax codes

• The 1263L tax code
• Scottish tax codes (the S prefix)
• Welsh tax codes (the C prefix)

This page is part of our tax rebate guides.

Code ending in W1, M1 or X? See W1 and M1 tax codes explained.