Millions of UK employees never check their tax code. HMRC believes that roughly one in six PAYE codes is wrong at some point in the year. That does not always mean you have paid too much tax, but it is a coin flip you should not be leaving to luck.
This is the short version of a tax code health check. It takes about ten minutes and could be worth hundreds of pounds.
Step 1: Find your current tax code
Your tax code appears on your most recent payslip, on your P60, and in any tax code notice HMRC has sent you (usually called a P2). If it does not match across those three, that alone is worth investigating.
Step 2: Compare it against the default
The default code for 2026/27 is 1257L. If your code is different, there is a reason. Common reasons include:
- A second job or pension (often BR, D0, or D1).
- Marriage Allowance transfers (M or N).
- Work benefits such as a company car or medical insurance.
- Underpaid tax from a previous year being collected (code ends in X or W1/M1).
- Adjustments for state pension or rental income.
Our tax code calculator explains each letter and shows how the code converts into a personal allowance figure.
Step 3: Sign in to your Personal Tax Account
Go to the GOV.UK Personal Tax Account. You can see the code HMRC currently has on file, the income sources HMRC knows about, and any adjustments that are being made. Match that against your own knowledge of your income.
Step 4: Run the arithmetic
Take the code number, multiply by ten, and that is your personal allowance for the year. So 1257L gives £12,570. Subtract that from your expected gross income for 2026/27. The remainder is what gets taxed at 20 per cent (up to £50,270), then 40 per cent above that.
Pair this with our salary calculator to compare the expected tax with what your payslip actually shows.
Step 5: Contact HMRC if it is wrong
If the figures do not line up, ring the HMRC income tax helpline on 0300 200 3300 or message them through your Personal Tax Account. Have your National Insurance number and a recent payslip ready.
HMRC can update the code within days. Your employer will normally use the new code on the next payday, and any overpaid tax is usually refunded through payroll over the remaining months of the tax year.
When to check again
Check again whenever your circumstances change — new job, pay rise, bonus, company car, pension top-up, second job, or a partner's income crossing a threshold. A five-minute check at those moments stops almost every common PAYE problem before it grows.