One wrong digit in your tax code and you could quietly overpay hundreds of pounds in tax, or face an unexpected bill at year-end.
Around one in three employees is on the wrong tax code at some point, according to tax specialists. HMRC issues millions of them every year, and most people never question the letters and numbers on their payslip. That silence can be expensive.
The numeric part of your tax code is simply your annual tax-free entitlement with the last zero dropped. For most UK employees in 2025/26, that is 1257, representing the standard Personal Allowance of £12,570. Your employer divides that by 52 (weekly pay) or 12 (monthly pay) and deducts tax only on what you earn above that slice.
If your code shows a number lower than 1257, something is eating into your allowance. Common culprits include untaxed company benefits (a car or private medical insurance), unpaid tax from a previous year being collected in instalments, or income from a second job. A higher number than 1257 is rarer but does happen, for example if you claim the Marriage Allowance transfer from a non-earning spouse, which adds up to £1,260 to your allowance in 2025/26.
The letter is where most of the confusion lives. It is not decoration; it tells payroll software which ruleset to apply alongside the number.
| Letter | What it means | Who typically gets it |
|---|---|---|
| L | Standard Personal Allowance applies | Most employees and pensioners |
| M | Marriage Allowance transferred IN (you received 10% of partner's allowance) | Lower-earning spouse |
| N | Marriage Allowance transferred OUT (you gave 10% of your allowance away) | Higher-earning spouse |
| T | HMRC needs to review your code; allowances are restricted or complex | Various |
| 0T | No allowance at all; all income taxed from the first pound | New job, no P45, or allowance fully used |
| BR | Basic rate (20%) on every penny; often a second job or pension | Second income sources |
| D0 | Higher rate (40%) on all income from this source | High earners with multiple sources |
| D1 | Additional rate (45%) on all income | Very high earners |
| NT | No tax deducted at all | Certain specific arrangements |
| K | Negative allowance; taxable benefits exceed the Personal Allowance | Company car users, for example |
The K code is especially worth understanding. Rather than a tax-free amount, a K code adds income to your pay before tax is calculated, because your benefits or unpaid tax exceed what your allowance can absorb. A code of K400 means HMRC is adding £4,000 of notional income to your earnings each pay period.
Start a new job without handing over a P45 from your last employer, and HMRC will often issue an emergency code. In 2025/26 that typically shows up as 1257L W1/M1 or simply 1257L X on your payslip. The allowance looks right, but the non-cumulative suffix means your employer cannot account for what you earned or paid tax on earlier in the year.
Suppose you left a job in June having already used £6,285 of your £12,570 allowance (half the year). You start a new job in July on £3,500 per month. Under a normal cumulative code, your employer would recognise you have already used half your allowance and apply the remaining £6,285 across the rest of the year, roughly £786 a month. Under a W1/M1 emergency code, they restart the clock and give you £1,047.50 of allowance per month (£12,570 divided by 12), which sounds better but ignores the real picture and can still leave you under- or over-taxed depending on your prior year earnings. You would need to contact HMRC or wait for an automatic end-of-year reconciliation (P800 notice) to get any refund. To avoid that wait, use the check my tax code tool to flag the issue early.
HMRC calculates your code based on information it holds: your P60s and P45s, employer reports under Real Time Information, your Personal Tax Account data, and any adjustments you have explicitly requested. Employers receive code notifications digitally and must apply them. Critically, HMRC does not always get it right, particularly when you change jobs, start receiving a company benefit, or have a side income that is assessed through Self Assessment.
For a quick sense of what your take-home should be on your current code, run your salary through the income tax and salary calculator to see the expected deduction and spot immediately if your payslip figure looks wrong.
If you are purely self-employed, HMRC does not issue a PAYE tax code for your trading income; you pay tax on profits through Self Assessment after the year ends. However, if you work part-time for an employer alongside running your own business, you will have a tax code for the employed portion. HMRC often adjusts that code to collect tax on your self-employment profits in real time, reducing your PAYE allowance to pre-collect what it estimates you will owe. That estimate is based on previous years and can be wildly off if your freelance income has changed significantly. Our full guide to UK tax codes covers how to handle this dual-income situation without ending up with a surprise bill.
Your tax code is HMRC's best guess at your situation; the burden is on you to tell them when the guess is wrong.
You do not need to wait for HMRC to fix an error. Log in to your Personal Tax Account on gov.uk, call HMRC's Income Tax line (0300 200 3300), or use the structured check my tax code walkthrough to identify what your code should be and how to request a correction. Changes mid-year are applied within the next payroll run once HMRC issues a new code to your employer, and any overpaid tax from earlier in the year is automatically refunded through payroll.
TapTax connects to your bank, categorises expenses automatically, and submits quarterly updates to HMRC. Free plan, no card required.
