Most people can tell you their salary to the pound and have no idea what their tax code is. That is understandable — it looks like an internal reference number, it arrives without explanation, and nothing about a payslip invites you to interrogate it. But the code is not administrative trivia. It is the single instruction your employer follows when deciding how much of your money to send to HMRC before you ever see it.
Get it wrong in one direction and you are lending the government hundreds of pounds a year for free. Get it wrong in the other and you are quietly building a bill that lands later, usually at the worst possible moment. Neither is rare, and neither is your employer's job to spot. Payroll applies the code it is given. Checking that the code is right is on you.
What the code is actually saying
A tax code has two parts, and each says something specific.
The numbers tell your employer how much tax-free income you get from them in that tax year. Take the number, add a zero, and you have the figure. The standard Personal Allowance for the 2026 to 2027 tax year is £12,570, which is why the most common code in the UK is 1257L. Your employer spreads that allowance evenly across your pay periods, so on monthly pay the first £1,047.50 of each month's pay is untaxed.
The letter explains the circumstances. These are the ones you are most likely to meet, in HMRC's own words:
- L — you get the standard tax-free Personal Allowance.
- M — Marriage Allowance: you have received a transfer of 10% of your partner's Personal Allowance.
- N — Marriage Allowance: you have transferred 10% of your Personal Allowance to your partner.
- T — your code includes other calculations to work out your Personal Allowance.
- 0T — your Personal Allowance has been used up, or you have started a new job and your employer does not have the details they need.
- BR — all income from this job or pension is taxed at the basic rate. Usually correct only if you have another job or pension using your allowance.
- D0 — all income from this job or pension is taxed at the higher rate. D1 — all of it at the additional rate.
- NT — no tax is being taken from this income.
- K — you have income that is not being taxed elsewhere and it is worth more than your Personal Allowance, so instead of tax-free pay you have an amount added to your taxable pay.
- S — Scottish rates apply. C — Welsh rates apply.
Then there are the emergency codes, which end in W1, M1 or X. These tax each pay period on its own instead of cumulatively across the year, which is why they so often take too much.
What a wrong code costs — the arithmetic
Take Priya, who earns £34,000 in a single job in England. Illustrative figures, but the mechanism is exactly how PAYE works.
On the right code, 1257L. The first £12,570 is covered by her Personal Allowance. The remaining £21,430 is taxed at 20%, which is £4,286 of Income Tax for the year, or about £357 a month.
On the wrong code, BR. BR means every pound from that job is taxed at basic rate with no allowance at all. That is £34,000 × 20% = £6,800.
The difference is £2,514 a year — £209.50 every month, gone from an account that could have used it. BR is not an exotic error either: it is the standard code for a second job, and it lands on people who started a new job without handing over a P45, or whose old employer never told HMRC they had left. If Priya only notices in month nine, she is £1,885 down, and while she will get it back, she has spent nine months without money that was always hers.
Now the other direction. A K code means untaxed income has outgrown your allowance. K475 adds £4,750 to Priya's taxable pay rather than sheltering anything, so she would be taxed as though she earned £38,750. If that code is right — a taxable company benefit, say, or tax owed from an earlier year being collected — then fine, it is doing its job. If it is based on a company car she handed back last spring, she is paying tax on a benefit she no longer receives, and only she will ever notice.
The rule of thumb: if you have one job, no company benefits and no untaxed income, your code should be 1257L. Anything else deserves an explanation you actually understand.
How to check yours — about five minutes
1. Find the code. It is on every payslip, on your P60 at the end of the tax year, and on a P45 when you leave a job. Look at the most recent payslip, not one from March.
2. Compare it with HMRC's version. Use the free Check your Income Tax for the current year service on GOV.UK, or the HMRC app. Both show your code and — more usefully — the workings behind it.
3. Read the breakdown, not just the number. This is the step people skip and it is the one that finds the error. HMRC builds your code from a list of assumptions: your jobs and pensions, any taxable benefits, any untaxed income it expects, any tax owed from a previous year. A wrong code is almost always a wrong item in that list. The usual suspects are a job you left still showing as current, a company car or medical insurance you no longer have, and an estimate of savings interest or rental income from a year that no longer resembles this one.
4. Correct it in the same place. If an entry is out of date you can update it in the service, and HMRC will issue a revised code to your employer. If your situation is harder to explain, the Income Tax helpline is 0300 200 3300, Monday to Friday, 8am to 6pm.
5. Check it again after anything changes. Starting or leaving a job, a second job, a bonus that pushes you into a new band, gaining or losing a company benefit, starting to draw a pension, or moving between England, Scotland and Wales — every one of these can change your code, and the change is not always applied correctly first time.
Claiming back what you have overpaid
If you have overpaid, HMRC often works it out itself and sends a tax calculation letter — a P800. Those go out between June and March following the end of the tax year, and the letter tells you how to get the refund.
You do not have to wait for one. If you think you have overpaid and no letter has come, you can claim through the Check how to claim a tax refund service on GOV.UK, which covers overpayments on pay from a job, on a pension, on a redundancy payment, on savings interest or PPI, and on job expenses such as working from home, uniform or tools.
The deadline is the part to act on. A claim must reach HMRC within four years of the end of the tax year it relates to. The 2022 to 2023 tax year ended on 5 April 2023, so a claim for that year has to be in by 5 April 2027. After that the money stays with HMRC permanently. If you have never checked, checking now covers the current year and four more.
And if the code was wrong in the other direction, the same logic applies with the sign reversed: an underpayment is usually collected by adjusting next year's code, spreading it across twelve months. That is far more comfortable than a demand, but it only happens if the problem is found early. The longer a wrong code runs, the bigger the correction.
One free thing most couples miss
If one of you earns below the £12,570 Personal Allowance and the other is a basic rate taxpayer — income between £12,571 and £50,270, or between £12,571 and £43,662 in Scotland — you can transfer £1,260 of allowance between you. That is Marriage Allowance, and it reduces the higher earner's tax by up to £252 a year.
You can see it working in the codes: the partner giving the allowance up gets an N, the one receiving it gets an M. Claims can be backdated to 6 April 2022 for any year you were both eligible, so a first-time claim can be worth several years at once. It costs nothing to claim, and it is done directly with HMRC — anyone charging you a percentage of it is charging you for something free.
Where to check the official positionTax code letters and what they mean are set out at GOV.UK: tax codes. Your own code and the workings behind it are in Check your Income Tax for the current year. Refunds are handled through Check how to claim a tax refund, and Marriage Allowance at GOV.UK: Marriage Allowance. For free, impartial money guidance backed by government, use MoneyHelper. The Personal Allowance of £12,570 and the 20%, 40% and 45% bands quoted here are the published figures for the 2026 to 2027 tax year.
Where coaching fits
Checking a tax code is not financial advice and it is not complicated — it is admin that nobody has ever sat you down and explained. Buzz Money Coach does the part that comes before any product decision: getting the facts straight, putting the numbers in order, and helping you decide what your money should do next. Coaching is not regulated financial advice, and we do not recommend pensions, investments or mortgages. Where a decision genuinely needs a regulated adviser we say so, and can introduce you to Equity & General, authorised and regulated by the FCA (No. 474163) — optional, with no obligation, and they pay us a commission if you proceed. If you are weighing up which kind of help you need, our guide on coaches and advisers sets out the difference.
A £209 monthly difference is not a small thing — it is an emergency fund, or a debt cleared a year earlier. If you want to see where a recovered overpayment would do the most good, the free Financial Freedom Score takes about seven minutes and gives you one clear next action. If you would rather have the whole picture written down and sequenced, that is what the Financial Roadmap is for.