How UK Tax Codes Work: A Payslip Decoder
Your tax code controls every penny HMRC takes from your salary. Here is exactly how UK tax codes work, what each letter means, and how to spot an error.
Your employer is not deciding how much tax to take from your wages. HMRC is. And they are doing it through a four-character string printed on your payslip that most people have never bothered to decode.
Understanding how UK tax codes work is not an accountant's hobby. It is the difference between paying the right amount of income tax and quietly handing HMRC hundreds of pounds every year that you are not legally required to pay.
- Your tax code tells your employer how much of your income to tax, not how much tax to take directly.
- The most common code, 1257L, gives you a £12,570 tax-free Personal Allowance for 2024/25.
- A wrong tax code can cost you hundreds of pounds per year, and HMRC will not always flag the error unprompted.
- You can check your current tax code for free at /check-my-tax-code in under two minutes.
- Letters in your tax code carry specific meanings: L, M, N, K, BR, D0, NT, W1, M1 all signal very different tax situations.
What a Tax Code Actually Does
- UK Tax Code
- A code issued by HMRC to your employer or pension provider that instructs them how much of your income to leave untaxed before applying income tax rates. It is made up of numbers and one or more letters, each carrying a specific meaning about your tax situation.
Here is the mechanism, stripped of jargon. Your employer runs payroll each month. They need to know how much income tax to deduct from your gross salary before it lands in your account. HMRC does not send them a bill; instead, HMRC sends a tax code, and your employer runs the maths themselves using that code as their instruction manual.
The number in your code, multiplied by ten, gives your tax-free allowance for the year. The letter tells your employer how to apply that allowance, or whether to ignore it entirely.
So 1257L, the code held by the majority of UK employees in 2024/25, means: leave the first £12,570 of this person's annual income untaxed, then apply the standard income tax rates to the rest. At a salary of £50,000, that produces a tax bill of £7,486. At £80,000, it produces £19,432. Get the code wrong, and both of those figures change, often upward.
The Number: Your Personal Allowance in Disguise
StriP the letter off your tax code and multiply the remaining digits by ten. That is the income you can earn before HMRC charges a single penny of income tax.
For most employees, that number is 1257, giving £12,570. But several things can make that number smaller, or even push it into negative territory.
When your allowance shrinks
If you have untaxed income elsewhere, such as rental income, savings interest above your savings allowance, or a company benefit like a car or medical insurance, HMRC reduces your tax-free number to collect the additional tax through your payslip rather than issuing a separate bill.
Example: you receive a company car valued at £5,000 in taxable benefit. HMRC reduces your code from 1257 to 757, leaving only £7,570 untaxed. Your employer then deducts income tax on the extra £5,000 across your twelve monthly payslips. Tidy in theory; catastrophic if the benefit figure has been calculated incorrectly.
When your allowance grows
You can also have more than £12,570 untaxed if you claim allowable expenses through your tax code, such as professional subscriptions, uniform cleaning allowances, or working from home relief. HMRC increases your code number to reflect this. A higher number means less tax deducted each month. If you are entitled to these reliefs and have never claimed them, you are overpaying quietly.
If you also do self-employed work alongside your PAYE salary, your code may also carry adjustments from your Self Assessment return, adding further complexity.
The Letter: Where Things Get Interesting
The letter is the most misunderstood part of how UK tax codes work. Each one carries a distinct instruction to your employer.
L
The most common letter. It confirms you are entitled to the standard Personal Allowance. Nothing unusual. If you have 1257L, HMRC considers your situation straightforward.
M and N
These relate to the Marriage Allowance. M means you have received 10% of your spouse or civil partner's Personal Allowance (boosting yours to £13,830 and saving up to £252 in tax per year). N means you have transferred 10% of your allowance to your partner. If you are eligible for Marriage Allowance and neither code appears on your payslip, you may be missing a legitimate tax break.
K
A K code means your deductions, such as taxable benefits or unpaid tax from a previous year, exceed your Personal Allowance. HMRC effectively adds the excess to your taxable income rather than subtracting it. K codes can produce unexpectedly high tax deductions and deserve close scrutiny. If you see a K on your payslip, verify with HMRC why your deductions are so large.
BR
Basic Rate only. Every penny of income from this source is taxed at 20%, with no Personal Allowance applied. This is correct if this is a second job and you have used your full allowance elsewhere. It is deeply wrong if it is your only income. We covered this in detail in BR Tax Code Meaning UK: You Are Paying 20% on Everything.
D0 and D1
All income from this source taxed at the higher rate (40%) or additional rate (45%) respectively. Correct only for very high earners with multiple income sources. Incorrect for the vast majority of people who see them.
NT
No Tax. HMRC has instructed your employer to deduct nothing. This is legitimately issued in specific circumstances, such as for non-UK-domiciled individuals in certain situations. If you have NT on your payslip and nobody has explained why, query it immediately.
W1 and M1
These are emergency codes. W1 means your tax is calculated weekly as if each week is the first of the year. M1 means the same but monthly. Both prevent HMRC from using the cumulative method that adjusts your tax if you overpaid or underpaid earlier in the year. Emergency codes are temporary and are typically issued when HMRC does not yet have your full income information. If yours has been sitting as W1 or M1 for more than a couple of payslips, something has not been updated.
0T
Another emergency code. All your Personal Allowance has been used up or HMRC has no information to issue a proper code. Tax is deducted at the relevant marginal rate with no allowance applied. Bad news for your take-home pay.
How HMRC Decides Your Code
HMRC does not conduct an annual interview with you before issuing your tax code. They make their best guess based on information they already hold: your previous year's tax return (if you file one), employer returns, pension provider data, and benefit-in-kind P11D submissions from employers.
That guesswork is frequently wrong. HMRC's own figures show millions of tax code adjustments are issued each year. Common triggers for errors include:
- Starting a new job without a P45 from your previous employer
- Receiving a company benefit that your employer reported at the wrong value
- Having income from multiple sources that HMRC has misallocated between your codes
- A previous underpayment that HMRC is collecting via your code without telling you clearly
- Life changes (marriage, divorce, pension access, rental income) that HMRC has not been informed about
None of these errors are your fault. But every one of them becomes your problem if you do not catch them.
The Personal Allowance Taper Nobody Mentions
If your income exceeds £100,000, something uncomfortable happens. Your Personal Allowance is reduced by £1 for every £2 you earn above that threshold. At £125,140, your Personal Allowance has been entirely withdrawn.
This means someone earning £120,000 has a tax-free allowance of only £7,570, not £12,570. Their tax code reflects this with a lower number. If you earn between £100,000 and £125,140 and your code still shows 1257L, HMRC has not updated your code, and you may face a significant underpayment at year end.
For higher earners, this taper also creates an effective marginal tax rate of 60% on income between £100,000 and £125,140. That is not a typo. It is the result of simultaneously losing Personal Allowance and paying 40% income tax on the income that replaced it.
If this bracket applies to you and you also have income beyond your PAYE salary, you may want to read about how multiple income sources interact with your tax code.
The Cumulative vs Week 1/Month 1 Distinction
Most employees are taxed on a cumulative basis. This means your employer considers your total earnings from the start of the tax year when calculating each month's deduction. If you were overtaxed in April, your employer automatically corrects it in May. The system is self-adjusting throughout the year.
Week 1 and Month 1 codes (marked W1 or M1) break this mechanism. Each pay period is treated as standalone. If HMRC issues an emergency code halfway through the year, you lose the benefit of any cumulative corrections, which can mean either overpaying tax for several months or, in the opposite scenario, building up an underpayment you will have to settle later.
This is one reason why an emergency code that sits unresolved costs real money. A higher earner on a M1 emergency code for six months could easily overpay £500 or more before the situation is corrected.
Reading Your Payslip Alongside Your Code
Your payslip should show your tax code, the period it covers, your gross pay, and the income tax deducted. Cross-referencing these takes thirty seconds and can reveal a problem immediately.
Divide your annual gross salary by twelve (or by 52 for weekly pay). Subtract the monthly personal allowance equivalent (£12,570 divided by twelve equals £1,047.50). Apply the relevant tax rate to what remains. Compare that figure to the tax deducted on your payslip.
If the numbers are materially different and you have not changed jobs, received a new benefit, or had an unusual income event recently, your code may be wrong. You can verify it in minutes at /check-my-tax-code.
If your code is wrong and you have been overpaying, HMRC can issue a refund through your payslip or via a P800 letter after the tax year ends. The process for getting that money back is covered in Wrong Tax Code? How to Fix It and Claim Back What HMRC Owes You.
People also ask
How to Check and Challenge Your Tax Code
HMRC provides a Personal Tax Account at gov.uk where you can view your current tax code and the calculation behind it. You can also call HMRC's Income Tax helpline on 0300 200 3300, though call wait times are routinely long.
If you believe your code is wrong, you do not need to accept it. You can notify HMRC of the error and request a corrected coding notice. Your employer will then receive an updated code, usually within a few weeks.
The fastest starting point is to run your current code through a check today. Visit /check-my-tax-code, enter your details, and see whether what HMRC is collecting matches what they should be collecting. It is free, it takes two minutes, and for a significant number of PAYE employees it surfaces an overpayment they did not know existed.
Your payslip has been telling you a number every month. Now you know exactly what that number means, and whether HMRC has earned the right to it.
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