UK Tax Codes Explained: From 1257L to K Codes & Beyond
Your tax code is the single most important driver of your monthly payslip. Unlike generic guides, this article is built around the exact parsing logic used in our 2026/27 calculator — covering Scottish prefixes, Marriage Allowance suffixes, negative K allowances and the 50% statutory cap.
Anatomy of a Standard Code: 1257L
The most common code for 2026/27 is 1257L. Breaking it down is simple, but the implications are easy to misunderstand.
- 1257 → Your tax-free Personal Allowance is £12,570 (1257 × 10).
- L → You are entitled to the standard tax-free amount with no special conditions.
In our calculator engine, this is parsed as pa = 1257 × 10 = 12570. The numeric part is always multiplied by 10, and the suffix determines how HMRC applies it.
Regional Prefixes: S, C & N
A prefix letter before the number forces the calculator to override the Region dropdown. This is critical because Scottish income tax has six bands instead of three.
| Prefix | Region Applied | Impact |
|---|---|---|
| S | Scotland | Uses Scottish bands: Starter 19%, Basic 20%, Intermediate 21%, Higher 42%, Advanced 45%, Top 48% |
| C | Wales | Currently identical to England for 2026/27, but the engine treats it separately for future-proofing. |
| N | Northern Ireland | Follows England rates and bands. |
Marriage Allowance: M & N Suffixes
When one partner earns below the Personal Allowance (£12,570), they can transfer £1,260 of it to the higher earner. HMRC bakes this directly into the tax code.
| Suffix | Meaning | Resulting PA | Tax Impact |
|---|---|---|---|
| M | Receiver of Marriage Allowance | £13,830 (£12,570 + £1,260) | Code already priced in; calculator skips the £252 double-count |
| N | Transferor of Marriage Allowance | £11,310 (£12,570 − £1,260) | Gives up £1,260 of allowance; partner saves £252 tax |
This means if your code ends in M or N, you should leave the Marriage Allowance dropdown on "None" in our calculator. The benefit is already in your code.
Flat-Rate & Emergency Codes
These codes bypass the Personal Allowance entirely and tax all income at a fixed rate. They are common for second jobs, company directors, or emergency tax situations.
| Code | Rate | When It Applies |
|---|---|---|
| BR | 20% | Second job; all income taxed at Basic Rate with no allowance. |
| 0T | 0% allowance, progressive | No Personal Allowance; income taxed through all bands (20%, 40%, 45%) from £0. |
| D0 | 40% | All income taxed at Higher Rate — often for high earners with multiple incomes. |
| D1 | 45% | All income at Additional Rate. |
| D2 | 45% | Additional Rate variant used in specific HMRC configurations. |
| NT | 0% | No tax deducted — rare, usually for non-residents or special exemptions. |
Scotland's Special Codes: SD0 – SD3, SBR, S0T
Scottish taxpayers have their own flat-rate emergency codes. Our calculator treats these as flatRate values with pa = 0.
| Code | Rate | Scottish Band Equivalent |
|---|---|---|
| SD0 | 21% | Intermediate Rate |
| SD1 | 42% | Higher Rate |
| SD2 | 45% | Advanced Rate |
| SD3 | 48% | Top Rate |
| SBR | 20% | Scottish Basic Rate (emergency) |
| S0T | Progressive | No allowance; Scottish bands from £0 |
If you are on SD1, every pound you earn is taxed at 42% plus National Insurance. There is no tax-free buffer. This is why checking your code matters so much around April each year.
K Codes: Negative Personal Allowance
A K code means you owe tax from previous underpayments, taxable benefits, or other adjustments. Instead of a tax-free allowance, you get a negative allowance that adds to your taxable income.
Example: K100 means your tax-free allowance is minus £1,000. If your gross salary is £40,000, HMRC treats it as £41,000 for tax purposes.
W1, M1 & X: Non-Cumulative Codes
Suffixes like W1 (week 1), M1 (month 1) or X tell HMRC to treat each pay period in isolation. You only get 1/52 or 1/12 of your allowance per period, and no under/overpayment balancing occurs.
Our calculator strips these suffixes when parsing the code (they do not change the numeric allowance), but they explain why your first payslip at a new job might look different from your final annual projection.
Quick Reference: 2026/27 Tax Codes
| Code | PA | Flat Rate | Region |
|---|---|---|---|
| 1257L | £12,570 | — | England / Wales / NI |
| S1257L | £12,570 | — | Scotland |
| 1383M | £13,830 | — | MA Receiver |
| 1131N | £11,310 | — | MA Transferor |
| K100 | −£1,000 | Progressive | Any |
| BR | £0 | 20% | Any |
| D0 | £0 | 40% | Any |
| D1 | £0 | 45% | Any |
| SD1 | £0 | 42% | Scotland |
| NT | ∞ | 0% | Any |