How tax codes are structured
A typical UK tax code has three parts: an optional country prefix, a body that defines your allowance, and an optional emergency-basis suffix.
In the code S1257L W1, for example:
- S — country prefix (Scotland)
- 1257 — the number, representing the personal allowance (£12,570 — multiply by 10)
- L — the suffix letter (standard allowance)
- W1 — emergency-basis suffix (each pay period treated standalone)
Most UK employees see a simple two-part code like 1257L without prefix or suffix.
What the number means
The number is your personal allowance divided by ten. So 1257 means £12,570 of tax-free annual income. If you've transferred part of your allowance to a spouse via Marriage Allowance, you'd see 1131N (£11,310). If you've received a portion from a spouse, you'd see 1383M (£13,830).
A code starting with K indicates a negative allowance — instead of receiving tax-free income, additional income is being added to your salary for tax purposes (typically when you have taxable benefits or owe HMRC tax from a previous year). K475 means £4,750 of extra taxable income is being added.
What the letter means
The suffix tells HMRC and the employer how to apply the allowance:
- L — standard personal allowance (most common)
- M — Marriage Allowance recipient (+£1,260 above standard)
- N — Marriage Allowance giver (−£1,260 below standard)
- T — special calculations apply (e.g., personal allowance taper above £100,000)
- 0T — no personal allowance (typically used above £125,140)
- BR — all income taxed at 20% (basic rate, no allowance — common for second jobs)
- D0 / D1 — all income at 40% / 45% (higher / additional rate flat codes)
- NT — no tax (rare exemptions)
Country prefixes
Tax bands differ across the UK. Your prefix tells your employer which bands to use:
- S — Scotland (Scottish bands apply)
- C — Wales (rUK bands apply — Wales hasn't varied rates yet)
- (no prefix) — England and Northern Ireland (rUK bands)
Emergency-basis suffixes
Codes ending in W1, M1, or X are on emergency basis — each pay period is calculated standalone rather than cumulatively across the year. This usually happens when:
- You started a new job without a P45
- HMRC is updating your code mid-year
- You've changed jobs recently
Emergency basis isn't punitive — it just means PAYE hasn't caught up to your year-to-date position. You'll usually be moved to cumulative basis automatically within a few pay periods, with any over-payment refunded.
When tax codes go wrong
The most common reasons for a wrong tax code:
- HMRC has the wrong information about your benefits in kind (company car, private medical)
- You have two jobs and HMRC has assigned BR to the wrong one
- You're recently retired and still on a pre-retirement code
- A previous-year underpayment is being collected when it shouldn't be
The fastest fix is the HMRC Personal Tax Account at gov.uk/personal-tax-account, where you can correct most issues directly.