UK tax code decoder

Type a tax code, get what it actually means — the allowance it grants, the rate it applies, and whether it is an emergency code. England, Wales and Scotland.

Live: https://hadidevlabx.github.io/uk-tax-code/

One static page. No framework, no build step, no dependencies, no tracking.

Codes it handles

Form	Example	Meaning
Number + letter	1257L 1383M 1131N 1257T	Allowance = number × 10
No allowance	0T	Allowance gone or unknown; normal bands still apply
Flat rate	BR D0 D1	Whole income at one rate
Negative allowance	K475	£4,750 added to taxable pay, 50% overriding limit
No tax	NT	Nothing deducted
Region	S1257L C1257L	Scottish / Welsh rates
Emergency	1257L M1 1257L W1 1257L X	Non-cumulative

The bit worth knowing

D0 means 40% in England but 21% in Scotland. D-codes count bands above basic, and Scotland has six bands to England's three, so the same letter lands somewhere else entirely. The decoder resolves them by band index rather than by hardcoded rate, which is why SD0→21%, SD1→42%, SD2→45%, SD3→48%.

That is also the case most decoders get wrong, so it has its own assertions in test.mjs.

Rates

Fetched at page load from uk-tax-rates, a free CC BY 4.0 dataset of UK income tax bands, NI thresholds and student loan plans. The page ships fallback figures and works offline if the fetch fails.

Test

node test.mjs

It extracts the real script out of index.html and asserts against it, so the test cannot drift from what ships.

Take it further

This explains the code. To apply one to an actual salary, see the tax code checker on True Take-Home.

Not tax advice. Your HMRC coding notice is the authority on your own code.

MIT licensed.