K tax code: what it means and how to check it's right

In short

Most tax codes give you a tax-free amount. A K code does the opposite: it tells your employer to add an amount to your taxable pay. HMRC uses it when your deductions (things you owe tax on that aren't taxed at source) are bigger than your tax-free Personal Allowance.

K codes are correct for some people. They're also one of the codes most likely to be wrong, because they depend on estimates HMRC makes about your income and benefits. If yours is wrong, you'll overpay every payday until it's fixed.

Why HMRC gives you a K code

Common reasons:

• Company benefits worth more than your allowance, such as a company car, fuel or private medical cover (these show on your P11D).
• State Pension plus a work or private pension, where the State Pension uses up your allowance.
• Untaxed income HMRC is collecting through your pay, such as rental income, savings interest or self-employed profits.
• Tax you underpaid in an earlier year that HMRC is collecting through your code.

How to read a K code

Take the number and multiply by ten, roughly. K475 means about £4,750 is added to your taxable pay across the tax year, instead of any tax-free amount.

A K at the front of a Scottish code shows as SK (for example SK475), and a Welsh one as CK. See Scottish and Welsh tax codes.

The 50% safety limit

Your employer can't take more than half of your pay in tax in any pay period because of a K code. Anything over that limit carries forward, so a K code can't wipe out a pay packet, but it can still take more than you owe over the year if HMRC's figures are wrong.

Common K code mistakes

1. A company car or benefit you no longer have is still being counted.
2. Income HMRC estimated too high, such as savings interest or rental profit that has fallen.
3. An old underpayment collected twice, or collected when it had already been paid.
4. State Pension counted in full when your circumstances changed partway through the year.
5. Expenses you're entitled to that aren't counted, such as uniform, professional fees or work mileage. Each one reduces the K amount.

How to check and fix it

1. Open your HMRC personal tax account or the HMRC app and look at "How we worked out your tax code". You'll need a Government Gateway user ID.
2. Check each line against your P60, your P11D and your own records.
3. Update anything wrong in the account. HMRC sends your employer a new code, and overpaid tax usually comes back through your pay in the same tax year.

Can I claim back tax from a wrong K code?

Yes. If a wrong K code made you overpay in earlier years, you can claim back up to four previous tax years. Right now that goes back to 2022/23, with a deadline of 5 April 2027 for that year. See How far back can you claim? No refund, no fee: see fees.

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Questions

Is a K tax code bad?

Not necessarily. It's correct if you genuinely have benefits or untaxed income worth more than your allowance. It's only a problem when the figures behind it are wrong.

Why has my tax code changed from 1257L to K?

Usually because HMRC has added a benefit, a pension or an underpayment to your record. Check "How we worked out your tax code" in your personal tax account. See Tax codes explained.

Can a K code take all my wages?

No. The 50% limit means no more than half your pay in any period can go in tax because of a K code.

How long will I be on a K code?

Until the reason for it ends or HMRC updates your record, often at the start of a new tax year.