What the numbers usually mean
GOV.UK explains that the numbers in a tax code tell your employer or pension provider how much tax-free income you get from them in that tax year. HMRC starts from your Personal Allowance and takes off untaxed income and other deductions such as some company benefits, then replaces the final digit of that tax-free amount with a letter. In practice, the number you see multiplied by ten is the usual tax-free amount: 1257 means £12,570. A different number, such as 1100 in HMRC’s medical-insurance example, means a lower tax-free amount of £11,000 from that job.
K codes work the other way around. If income you are not paying tax on is more than your Personal Allowance, a K code adds the number times ten to your taxable pay from that employment or pension, rather than giving you a tax-free amount. This decoder accepts both K497 and 497K and will say so clearly.
Letters, prefixes and emergency suffixes
Letter meanings on this page follow the official “what your tax code means” table. L is the standard Personal Allowance. M and N are Marriage Allowance (received or transferred). T means other calculations are included. BR, D0 and D1 tax all income from that job at the basic, higher or additional rate, and are often used when you have more than one job or pension. NT means no tax is taken from that income. 0T means the Personal Allowance has been used up, or a new employer does not have the details needed for a fuller code.
An S prefix means Scottish rates apply to that income. A C prefix means the rates that apply in Wales, which are the same main rates as England and Northern Ireland. Scottish letter-only codes such as SD0 (intermediate), SD1 (higher), SD2 (advanced) and SD3 (top) are recognised with the official wording.
You are on an emergency tax code if the code ends in W1 (weekly), M1 (monthly) or X (variable pay dates), or if a payslip shows NONCUM. Official employer pages state that the emergency tax codes from 6 April 2025 and from 6 April 2026 are 1257L W1, 1257L M1 and 1257L X. Emergency tax is worked out on that week or month only, as if you were paid the same amount every week or month of the year, which can mean you pay too much or too little until HMRC issues an updated code. M1 as a suffix is not the same thing as the letter M for Marriage Allowance; 1257L M1 is emergency monthly, while 1383M would be a Marriage Allowance letter with a £13,830 tax-free amount.
What this decoder will not do
It will not say that a decoded code is definitely your correct code. Only HMRC and the payroll records they issue can do that. It will not work out the tax on your pay, apply company benefits, or tell you whether you should be on a Scottish or Welsh prefix. If you think the code is wrong, use the official tax-code service on GOV.UK or contact HMRC, and keep your payslips.