Your tax code is on your payslip, in the HMRC app and in HMRC's check your Income Tax service, which also shows the sum behind it. To test it, take your Personal Allowance, subtract anything HMRC is collecting through your pay, and compare the result with the number in your code multiplied by ten. The two should be close: HMRC drops the last digit when it builds the code, so a few pounds apart is normal.
Where to find the code
You get a code for each employment and each pension you have. Find them all before judging any of them, because a code only makes sense next to the others.
GOV.UK lists four places to look: the check your Income Tax service online, which also holds your codes for earlier years and can email you when a code changes; the HMRC app; your payslip; and a Tax Code Notice letter from HMRC, the form often called a P2.
Two other documents help without being on that list. Your P45, the form you get when you leave a job, shows the code your old employer was using. Your P60, which arrives at the end of the tax year, shows what you were paid and taxed, and that is what you need to test whether the code did its job.
The two-minute check
The number in your code is the tax-free pay you get at that job for the year, with the last digit dropped. So work out the sum HMRC should be doing, then compare.
Start with the Personal Allowance, £12,570 for 2026/27. Add anything that increases it: a Marriage Allowance transfer you have received, or job expenses HMRC has agreed. Take off the taxable value of company benefits, untaxed income HMRC knows about, and any tax from an earlier year being collected through your code.
Now multiply the number in your code by ten and see whether the two agree. GOV.UK's wording is that you usually multiply by ten. Because HMRC drops the last digit of your tax-free pay when it builds the code, the two figures can sit a few pounds apart with nothing wrong.
If the answer goes below zero you get a K code instead, and the same arithmetic runs in reverse. The check your Income Tax service shows HMRC's own version of this list, item by item, which is the fastest way to see which entry is off.
Then test the outcome rather than the code. On £35,000 with the standard code, a full year comes to £2,393.34 a month in take-home pay and £373.83 a month in income tax. If your payslip is far from that, something in the code, the pay period or the year-to-date figures is wrong.
Six signs a code needs attention
BR or D0 on your only job. Both codes assume your allowance is being used elsewhere. 0T on your only job is the same signal, unless your adjusted net income is high enough for the Personal Allowance to have tapered away, in which case 0T is the correct code.
W1, M1 or X months after you started. An emergency code should not still be running once HMRC has your details.
A code that has not changed after a benefit ended. A company car handed back stays in your code until HMRC is told about it.
An S or C prefix that does not match where you live. That letter is the rate ladder, and it follows your main home rather than your employer.
Two codes that both look like main-job codes. Only one job should hold your full allowance.
A K code you cannot explain from the items on your coding notice.
How to get it changed
Sign in to the check your Income Tax service and work through your employments, pensions, estimated taxable income, company benefits and expenses. Correct anything wrong or missing there, because that is what HMRC uses to rebuild the code.
If you have left a job without a P45, ask the employer for one. It carries the pay and tax figures HMRC is waiting for. If you cannot use the online service, contact HMRC. After starting a new job, GOV.UK asks you to allow 35 days for your income details to reach HMRC before getting in touch.
What happens next
When the code changes, HMRC tells you and your employer within 15 working days. On a monthly payroll the new code should appear on your next payslip or the one after; on a weekly payroll, by the third. If your payslip still shows the old code after that, ask your employer whether they have received it.
Any overpaid tax comes back through your pay once the new cumulative code is applied, provided HMRC holds your income details for the year. If it does not, the correction waits until after 5 April, when HMRC checks the year against what your employers reported and writes to you.
How often should I check it?
Codes change at predictable moments, and a code that has not caught up with a change is the one that goes wrong. New codes go out before 6 April for the year ahead. After that, HMRC rebuilds a code when something it holds about you changes: a new job, a second job, a company benefit starting or ending, untaxed income, a pay rise past a threshold, or a move to or from Scotland or Wales.
Can my employer change my tax code for me?
No. Employers apply the code HMRC sends and cannot amend it on request. They can tell you which code they are using and when they received it, which is useful when a correction seems to have gone missing, but the change itself has to come from HMRC.