Your tax code is probably wrong if you're being taxed more (or less) than you expect, or if your code isn't 1257L without a clear reason. The quickest way to know for certain is to sign in to HMRC's free Check your Income Tax service, compare your code and income against your payslip, and update anything that's out of date. If HMRC has the wrong figures, your code will be wrong too, and you'll either overpay or underpay tax until it's fixed.
What does my tax code mean?
A tax code tells your employer or pension provider how much of your income is tax-free. For 2026/27 the standard Personal Allowance is £12,570, so the most common code is 1257L, the numbers are your allowance divided by 10, and the letter describes your situation.
- L, you get the standard Personal Allowance.
- BR, all income from this job is taxed at the basic rate (20%), usually because it's a second job.
- D0 / D1, taxed at higher (40%) or additional (45%) rate, again often a second income.
- 0T, no tax-free allowance is being applied.
- K, deductions (like company benefits) exceed your allowance, so tax is added.
- W1 / M1 / X, emergency codes applied on a "week 1/month 1" basis.
The thresholds your code works against in 2026/27: basic rate up to £50,270, higher rate from £50,271, and the additional rate from £125,140, at which point your Personal Allowance has already tapered to nil (it starts reducing once income passes £100,000).
How do I know if my tax code is wrong?
Look for these red flags:
- You've started a new job and were put on an emergency code (1257L W1/M1, or 0T).
- You have two jobs or a pension and your allowance is split oddly, or applied twice.
- A company benefit (car, medical insurance) has started or ended but your code hasn't moved.
- You're seeing a K code you don't understand.
- Your take-home pay suddenly jumped or dropped with no change to your salary.
- You repaid a company benefit or stopped claiming something, but the deduction is still there.
A quick sense-check: drop your salary and tax code into our take-home pay calculator and compare the result with your actual payslip. If they don't line up, your code is the usual culprit.
Why emergency tax codes happen
Emergency codes (like 1257L W1/M1) are temporary. They're applied when HMRC doesn't yet have full details, typically a new starter without a P45. You still get the £12,570 allowance, but it's calculated each period in isolation, so it often over-taxes you until your real code arrives. It usually corrects itself once your employer reports your details, but it's worth chasing if it drags on.
How to check your tax code
Book a free Tax Health Check →
- Find your code. It's on your payslip, your P45/P60, or any HMRC tax code notice (form P2).
- Sign in to HMRC. Use the Check your Income Tax for the current year service with your Government Gateway login.
- Check the assumptions. HMRC builds your code from an estimated annual income, plus deductions for benefits or untaxed income. If those estimates are wrong, your code is wrong.
- Compare to reality. Match HMRC's figures to your actual employment, pension and benefit details.
How to get a wrong tax code fixed
If something is out of date or missing, you can correct it yourself online, this is the fastest route. HMRC's official guidance is here: If you think your tax code is wrong.
Update it online
Sign in to the Check your Income Tax service and update your employment, estimated income, company benefits and expenses. HMRC will recalculate your code, send a new notice to you and your employer, and the change usually flows through to your next pay run or the one after.
If you can't use the online service
Phone HMRC's Income Tax helpline (0300 200 3300) with your National Insurance number and payslip to hand. In most cases HMRC updates your code automatically when your employer reports a change, so a wrong code often points to outdated information rather than a system error.
Will I get a refund if my tax code was wrong?
Yes, if a wrong code made you overpay, you're due that money back. Where the error is corrected within the same tax year, the refund typically comes through your payslip automatically as your code adjusts. If it spans a previous tax year, HMRC reconciles it after year-end and issues a P800 calculation, after which you can claim the repayment online or by bank transfer.
If you underpaid, HMRC will usually recover it by adjusting next year's code rather than asking for a lump sum, though you can pay it directly if you'd prefer to clear it.
How long does a tax code change take?
Once HMRC updates your code, it's normally reflected in your next one or two pay runs, depending on your employer's payroll cut-off. Online updates are processed faster than phone or post. Keep an eye on the next payslip to confirm the new code, and the corrected tax, has landed.
For more quick answers on PAYE, allowances and refunds, see our FAQ.
Tax codes are easy to get wrong and easy to overlook, and an incorrect one can quietly cost you hundreds over a year. If your code looks off, you've changed jobs, picked up a second income, or you're staring at a K code you can't decode, talk to a Zmartly accountant. We'll check it against your real figures, deal with HMRC for you, and make sure any overpaid tax comes back where it belongs.
