Tax Code Calculator & Checker

Tax & Income Guide

How UK tax codes work

Your tax code tells your employer or pension provider how much income to treat as tax-free each pay period. HMRC sends a tax code to your employer, who then uses it to calculate how much income tax to deduct through PAYE before your pay reaches your bank account. For a standard code like 1257L, the number (1257) multiplied by 10 gives your annual personal allowance (£12,570). The letter tells your employer about your specific circumstances.

Most people will only ever see one or two different tax codes in their lifetime. But when something changes, a new job, a second income, a company car, or a change in benefits, your code can shift. Understanding what yours means helps you catch errors before they cost you.

Tax code letters explained

The letter suffix in your tax code carries important meaning:

Scottish and Welsh tax codes

If you live in Scotland, your tax code starts with S (e.g. S1257L). This tells your employer to apply Scottish income tax rates, which differ from the rest of the UK, Scotland has more bands, starting with a 19% starter rate and topping out at 48% for the highest earners. If you live in Wales, your code starts with C (e.g. C1257L), applying Welsh rates. Welsh rates have been set at the same level as England and Northern Ireland since devolution, but Wales has the power to change them independently.

HMRC determines your residence from your address. If you move between Scotland, Wales, and England, it can take a pay period or two for your code to update, which can cause a brief over- or underpayment that HMRC will correct at year end.

Emergency and non-cumulative codes (W1/M1)

When you start a new job without providing a P45, or when HMRC has insufficient information, your employer may use an emergency tax code. These codes are often shown with W1 (weekly pay), M1 (monthly pay), or X (any pay period) after the number, for example, 1257L M1.

The critical difference: a normal tax code is cumulative, meaning your employer takes your unused allowance from earlier in the tax year into account. An emergency W1/M1 code is non-cumulative, each pay period is treated as standalone. If you received little or no income early in the tax year, this means you'll overpay tax because that earlier unused allowance is ignored. Once HMRC issues a correct code, the cumulative system catches up and you'll pay less tax in subsequent months to compensate.

Marriage Allowance and the M/N codes

Marriage Allowance lets you transfer £1,260 of your personal allowance to a spouse or civil partner who earns more, saving them up to £252 in tax per year. The person giving up their allowance receives an N code (reducing their personal allowance to £11,310); the partner receiving it gets an M code (increasing their allowance to £13,830). You can apply online through HMRC and the saving is applied in-year, not just at year end.

When a tax code goes wrong

Emergency codes (W1/M1), D0, or 0T codes can cause you to overpay tax during the year. HMRC's cumulative PAYE system usually corrects overpayments automatically at year end, but you can also claim a refund through your Personal Tax Account or by calling HMRC on 0300 200 3300. If you have been on an emergency code for more than a couple of pay periods, it is worth contacting HMRC directly to resolve it, particularly if you have already provided your P45 to your employer.

Common reasons for a wrong code include: starting a new job, receiving a taxable benefit (company car, private medical), having multiple income sources, or receiving the State Pension alongside employment income. HMRC's systems are mostly automatic, but they rely on accurate information, if your circumstances change, it is often quicker to update HMRC proactively rather than waiting for the system to catch up.

How to check and update your tax code

You can view and update your tax code through your HMRC Personal Tax Account at gov.uk/personal-tax-account. You'll need a Government Gateway login. From there you can see all your current tax codes across employers and pensions, check what HMRC thinks your income is, and request changes if something looks wrong. Changes made online are usually reflected in your next payslip once HMRC sends a new coding notice to your employer.

For complex situations, K codes, multiple income sources, untaxed state benefits, or high-income personal allowance tapering above £100,000, it is worth speaking to HMRC directly or consulting a tax adviser. HMRC's cumulative adjustments usually resolve overpayments automatically, but reclaiming an overpayment from a prior tax year requires a formal repayment claim.