Understanding Your Tax Code

How to read the letters and numbers on your payslip, and why a wrong one is a common reason people overpay or underpay without realising it

Your tax code is the short string of letters and numbers your employer uses to work out how much Income Tax to take from your pay, something like 1257L. It's easy to ignore, but it's worth understanding, because a wrong tax code is a common reason people overpay or underpay tax without realising it. You'll find it on your payslip and on your P60. It's also easy to confuse with your National Insurance number, a longer, permanent identifier that doesn't change from job to job the way your tax code can.

The standard code: 1257L

For most employees with one job and no unusual circumstances, the code for 2026/27 is 1257L. Here's what the two parts mean:

If your circumstances are simple, i.e. one job, no company benefits, no untaxed income, no unpaid tax from a previous year, then 1257L is very likely correct.

If your main home is in Scotland, your code gets an S prefix instead (e.g. S1257L) and is worked out against genuinely different Income Tax bands. See Scottish Income Tax Explained for how much that actually changes. A Welsh main home gets a C prefix (e.g. C1257L), but works exactly like an ordinary code underneath, since Wales hasn't set its own rates.

Other letters you might see

See Marriage Allowance for how the M/N transfer actually works and whether you'd benefit from it.

Decode your tax code

If yours doesn't look like the standard 1257L, enter it below to see what it actually means, rather than matching it against the table above yourself.

Why your tax code gets reduced

Your code doesn't stay at the standard 1257L forever. HMRC reduces the number in your code, giving you a smaller tax-free amount, whenever it knows about income you're receiving that isn't being taxed another way. The two most common reasons:

• Taxable company benefits. Things like private medical insurance, a company car, or interest-free loans from your employer are worth money to you but aren't paid as salary, so they can't be taxed through your payslip the normal way. Instead, HMRC (usually informed by your employer's P11D return) reduces your tax-free allowance by the benefit's value, so it effectively gets taxed through your code instead.
• Tax you underpaid last year. If you owe HMRC money from a previous tax year, say your tax code was wrong for a few months, or you had income that wasn't fully taxed, HMRC will often "code out" the debt rather than send you a bill: they reduce your allowance by enough that the extra tax collected over the year recovers what you owe. This only applies to underpayments below £3,000; anything larger is usually collected another way.

The two work slightly differently. A benefit's value comes straight off your allowance, pound for pound, because it represents actual untaxed income. An underpayment, by contrast, isn't an amount of income, it's a debt, so HMRC has to work backwards from how much extra tax they need to collect, using your tax rate. For a basic-rate (20%) taxpayer, recovering £1 of debt over the year requires reducing the allowance by £5, since 20% of £5 is £1.

If your reductions are big enough to wipe out your entire Personal Allowance and more, your code doesn't simply hit zero, it switches to a K code instead, covered below. HMRC usually tells you about a change like this via a P800, its end-of-year tax calculation letter.

What a "K" code means

A K code is the odd one out: instead of giving you a tax-free amount, it means HMRC has decided you owe more than your Personal Allowance can offset. The number gets added to your taxable pay instead of being deducted from it. For example, K285 means £2,850 gets added to your taxable income before tax is worked out, not subtracted from it. If you have a K code, it's worth understanding why. Check what's listed against it in your personal tax account or the HMRC app.

Emergency tax codes

If you start a new job and your employer doesn't yet have your previous pay and tax details (for example, you didn't have a P45 to give them), you may be put on an emergency tax code. For 2026/27, this is 1257L, the same as the standard code, but with a suffix:

• 1257L W1 or 1257L M1: "Week 1" or "Month 1" basis. Instead of working out your tax cumulatively across the year (accounting for what you've already earned and paid), each pay period is treated in isolation. This is usually temporary and gets corrected once HMRC has your full details, but it can mean you're taxed more than you should be in the meantime, especially if you started partway through the tax year after a period of lower or no income.
• 1257L X: used when there isn't enough information to determine which basis to use; similar effect to W1/M1.

Emergency tax usually sorts itself out within a pay period or two once your employer has your P45 or you've completed a starter checklist, but if it doesn't, it's worth contacting HMRC directly, since it won't always correct itself automatically. If you were overtaxed for a while before it was corrected, that overpayment often shows up the following year as a refund on a P800, HMRC's end-of-year reconciliation letter.

How to check your tax code is right

Your tax code is set by HMRC based on the information they hold about your income, benefits, and any allowances or adjustments, but it's not infallible, especially if you've changed jobs, started a second income source, or had a change in company benefits.

To check yours:

1. Find your current tax code on a recent payslip or your P60.
2. Check it against your personal tax account or the HMRC app, which shows what HMRC has on record and why.
3. If something doesn't match what you'd expect, for example, you're on BR in your only job, or a K code with no obvious explanation, contact HMRC to query it.

Getting this wrong isn't just a minor inconvenience: an incorrect code can mean months of over- or under-paid tax before it's noticed, particularly relevant if your income is near the 60% tax trap band, where HMRC doesn't always update your code automatically when a bonus or pay rise pushes you over £100,000.

This page explains general tax code rules for employees and is not personalised advice; see the disclaimer.