Your tax code is the small string of numbers and a letter printed near the top of your payslip, something like 1257L. It quietly controls how much tax-free income you get and therefore how much tax comes out of every pay packet. A wrong code is one of the most common reasons UK workers overpay tax for months without noticing, or get an unwelcome bill later. This guide explains how to read a tax code, what each part means, and how to tell whether yours is right. We focus on the structure rather than this year’s exact allowance figure.

What a tax code is actually doing

PAYE needs to know how much of your income to leave untaxed before it starts deducting. Your tax code is the instruction that tells your employer or pension provider exactly that. HMRC works out the code, sends it to your payer, and your payer applies it to each payslip.

In short: the tax code translates your personal tax-free allowance into a number your payroll software can use. Get the code right and PAYE deducts close to the correct amount automatically. Get it wrong and every payslip is off until it is fixed.

Reading the number

The numeric part of a standard code represents your tax-free allowance for the year, with the final digit removed. The rule is simple: multiply the number by 10 to get the tax-free amount.

So a code with the number 1257 corresponds to £12,570 of tax-free income, because 1257 times 10 is 12,570. A code numbered 1000 would mean £10,000 of allowance.

The number is not always the standard personal allowance. HMRC adjusts it up or down for things specific to you:

  • Increases for allowances you are owed, such as certain job expenses or the receiving half of the Marriage Allowance.
  • Decreases for things that reduce your tax-free room, such as taxable workplace benefits (a company car, private medical cover) or untaxed income HMRC is collecting through your code.

This is why two colleagues on the same salary can have different code numbers. The number is personalised.

To see the tax-free amount and rough deductions implied by a specific code, our tax code calculator does the conversion.

Reading the letter

The letter (or letters) at the end describes your situation and how the allowance should be applied. The common ones:

  • L: the standard code, you receive the basic personal allowance.
  • M: you have received a Marriage Allowance transfer from your partner, increasing your allowance.
  • N: you have given away part of your allowance to your partner under Marriage Allowance, reducing yours.
  • T: your code includes other calculations HMRC needs to review, often used in more complex cases.
  • 0T: you have no tax-free allowance left in this employment, every pound is taxed from the first.
  • BR: all income from this source is taxed at the basic rate, with no allowance. Common on a second job or pension, where your allowance is already used by your main job.
  • D0: all income from this source is taxed at the higher rate.
  • D1: all income from this source is taxed at the additional rate.
  • NT: no tax is to be deducted at all, used in specific circumstances.

A letter prefix of S means Scottish rates apply, and C means Welsh rates apply. So S1257L is the standard code for a Scottish taxpayer.

The K code: when deductions exceed your allowance

Most codes give you tax-free room. A K code does the opposite. It is used when the things reducing your allowance (large taxable benefits, or untaxed income being collected) are bigger than the allowance itself. Rather than a tax-free amount, a K code adds an amount to your taxable income.

In a K code, the number works in reverse: it represents extra taxable income to be added, again multiplied by ten. So payroll treats you as if you had additional income on top of your pay, and taxes accordingly. There is a built-in protection that stops a K code from taking more than half of a given payment in tax, which can spread the effect over time.

K codes are not a penalty, they are simply how PAYE collects tax on benefits or income that exceed your personal allowance without sending you a separate bill.

Emergency and non-cumulative codes

When you start a job and your new employer does not yet have full details (for example you have not handed over a P45), HMRC or the payer may use an emergency code. An emergency code usually gives you the basic allowance but applies it on a non-cumulative basis, often shown with W1 (week 1) or M1 (month 1) after the code.

The difference matters:

  • A cumulative code looks at your total pay and total allowance for the year so far each time you are paid, so it self-corrects. If you were overtaxed earlier, a later payslip can refund it automatically.
  • A non-cumulative code (W1/M1) treats each pay period in isolation, giving you a slice of allowance for that period only and ignoring earlier months. It cannot look back and correct over- or under-payment.

Because of this, an emergency code can over-withhold, especially if you had a gap in employment that left unused allowance earlier in the year. Once HMRC has your full details, your code normally switches to a cumulative basis and any overpayment is reconciled. Our take-home pay calculator lets you compare what a normal code should deduct against what you are actually seeing.

A worked example with round numbers

Suppose the standard allowance is £12,000, giving a standard code numbered 1200.

  • An employee with a £600 taxable benefit (say medical cover) has their allowance reduced to £11,400, so their code becomes 1140.
  • An employee receiving a Marriage Allowance transfer worth £1,200 has their allowance raised to £13,200, so their code becomes 1320, with the M suffix.
  • An employee with a £15,000 company car benefit, which exceeds the £12,000 allowance by £3,000, would be put on a K code reflecting that £3,000 of extra taxable income, written as K300.

The same salary produces different deductions in each case purely because the code, and therefore the tax-free room, differs.

How to check whether your code is right

A few quick checks catch most errors:

  • Does the number look plausible? If you have no special circumstances, it should be close to the standard personal allowance number. A much lower number means something is reducing your allowance, check whether that is correct.
  • Do you have an unexpected BR, 0T or D code on your main job? Those remove your allowance and are usually only correct on a second income. On your main job they often signal a missing P45 or an HMRC mix-up.
  • Are you still on a W1/M1 emergency code months into the job? It should usually have switched to cumulative once HMRC had your details.
  • Have your benefits changed? Giving up a company car or medical cover should raise your code number again.

If your code is wrong, contacting HMRC to correct it triggers an updated code to your employer, and PAYE then reconciles the difference. Where the year has ended, HMRC may issue a P800 calculation showing an over- or underpayment, which our P800 tax refund calculator helps you interpret. To check how a corrected code changes your overall tax, our income tax calculator shows the full-year position.

Frequently asked questions

What does the number in my tax code mean?

It is your tax-free allowance for the year with the last digit removed. Multiply it by ten to get the actual amount. A code numbered 1257 means £12,570 of income before tax begins. HMRC adjusts this number up for allowances you are owed and down for taxable benefits or untaxed income collected through your code.

Why am I on a BR or 0T code?

Both remove your tax-free allowance for that income source. They are normal on a second job or pension, where your main job already uses your allowance. On your main job they usually indicate a problem, such as a missing P45, and are worth querying with HMRC because you may be overpaying.

What is a K code and why do I have one?

A K code applies when the deductions from your allowance (large benefits or untaxed income) are bigger than the allowance itself. Instead of giving tax-free room, it adds taxable income. It is the way PAYE collects tax on those amounts through payroll rather than a separate bill. A protection limits it to taking no more than half of any single payment.

Why did I get taxed so heavily when I started a new job?

You were probably on an emergency, non-cumulative code (shown as W1 or M1) because your employer did not yet have your P45. A non-cumulative code ignores earlier months and cannot refund earlier overpayment. Once HMRC has your full details, the code normally switches to a cumulative basis and any overpaid tax is reconciled.

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