How to Check Your UK Tax Code (And Fix It If It Is Wrong)
An estimated 5 million UK workers are on the wrong tax code at any given time. An incorrect code can mean overpaying or underpaying tax — and HMRC will eventually collect any shortfall, often as a surprise deduction months later.
What is a tax code?
Your tax code tells your employer how much income tax to deduct from your pay each month via PAYE. The most common code for 2026–2027 is 1257L — the "1257" means you have a tax-free Personal Allowance of £12,570, and the "L" means you are entitled to the standard allowance. Around 5 million UK workers are estimated to be on the wrong tax code at any given time.
How to check your tax code
The easiest way is via your HMRC Personal Tax Account at gov.uk/check-income-tax-current-year. Sign in with your Government Gateway account. You can also see your tax code on your payslip (look for the "Tax Code" field), on your P60 at the end of each tax year, or on a PAYE coding notice (P2) that HMRC sends by post or online.
What the letters in your tax code mean
L = standard Personal Allowance (most common). M = you receive Marriage Allowance transferred from your partner (+£1,260). N = you have transferred Marriage Allowance to your partner (−£1,260). BR = all income taxed at 20% Basic Rate — common for a second job. D0 = all income taxed at 40% Higher Rate. D1 = all income at 45% Additional Rate. K = negative allowance — HMRC is collecting unpaid tax from previous years. NT = no tax deducted. S prefix = Scottish taxpayer. C prefix = Welsh taxpayer. W1 or M1 suffix = emergency non-cumulative code.
Why your tax code might be wrong
HMRC can issue an incorrect code for many reasons: your P45 was not processed when you changed jobs; you have a company car or other taxable benefit in kind; you have more than one employer; you underpaid or overpaid tax in a previous year; you recently started or stopped receiving a pension; or HMRC used an estimated income figure that was inaccurate. Each of these scenarios can result in under or overpayment of tax.
How to correct a wrong tax code
Log into your HMRC Personal Tax Account at gov.uk/check-income-tax-current-year and update your employment and income details. You can also call HMRC on 0300 200 3300 (Monday to Friday, 8am–6pm). Changes usually take effect in the next pay period, and HMRC will spread any over or underpayment across the remaining months of the tax year automatically.
Emergency tax codes — what to do
Starting a new job without a P45 may result in an emergency tax code ending in W1 or M1. This calculates tax non-cumulatively each pay period rather than accounting for the full year, which often leads to overpayment. Provide your P45 to your new employer as soon as possible, or contact HMRC directly. Any overpaid tax will be refunded, either through your payslip or via a P800 tax calculation at the end of the year.
How much could a wrong tax code cost you?
The cost depends on which direction the error goes. An emergency BR (Basic Rate) code on a second job can mean you pay 20% tax from the first pound of earnings instead of using your Personal Allowance — potentially hundreds of pounds per year. An incorrect benefit-in-kind estimate can similarly cause under or overpayment. It is worth checking your code every April at the start of the new tax year.
Calculate your take-home pay with the correct tax code
This guide is for general information only. Tax rules can change — always verify current rates with HMRC at gov.uk or by calling 0300 200 3300.