What Is a Tax Code and Why Does It Matter?

5.7m

Wrong tax code notices issued by HMRC annually, per NAO estimates

£252

Maximum annual saving from Marriage Allowance transfer (2026/27)

4 years

How far back you can reclaim overpaid tax from an incorrect code

Your tax code is the instruction HMRC sends to your employer telling them how much of your pay should be tax-free before deducting Income Tax under PAYE. Get it wrong — or let HMRC get it wrong — and you either overpay tax silently every month, or underpay and face an unexpected bill at year end.

The standard code for 2026/27 is 1257L. The number represents your annual tax-free amount divided by 10 (£12,570 Personal Allowance divided by 10 equals 1,257). The letter tells your employer how to apply that allowance. HMRC issues around 5.7 million incorrect code notices annually and most go unchallenged because workers do not understand what their code means.

How to read your tax code — the anatomy

1257

The number

Multiply by 10 to get your annual tax-free allowance: 1257 × 10 = £12,570. A higher number means more tax-free pay; a lower number means less.

+

L

The letter

Tells your employer how to apply the allowance. L = standard Personal Allowance. Different letters change the rate, method, or amount significantly.

Every tax code letter explained

Code	What it means	Who gets it	Watch out for
L	Standard Personal Allowance	Most employees and pensioners	Number should be 1257 in 2026/27 unless you have extra allowances or deductions
M	Marriage Allowance recipient — allowance increased by £1,260	Partner who received the transfer	Code should be 1383M. Still showing 1257L? The transfer has not been applied — you are missing £252/year.
N	Marriage Allowance transferor — allowance reduced by £1,260	Partner who gave the transfer	Code should be 1131N. Check your partner has M suffix — both need updating simultaneously.
T	Other adjustments; HMRC reviews annually	Complex situations: multiple employments, estimated income	Log in to your Personal Tax Account to see the components making up the T code.
0T	No Personal Allowance; taxed at marginal rates on all income	New starters without a P45; earnings above £125,140	Often applied in error. Resolve immediately — overpayment can be £2,514+/year at basic rate.
BR	Basic Rate on all income (20%) — no allowance	Second jobs; pensions where main source uses full allowance	If this is your only income source, it is almost certainly wrong. You are overpaying ~£2,514/year on £35,000.
D0	Higher Rate on all income (40%)	Second jobs where higher-rate tax is due on additional income	Wrong if applied to sole employment — contact HMRC immediately.
D1	Additional Rate on all income (45%)	Third or further employments for very high earners	Rare. Should only apply if all other income is already in the additional rate band.
K	Negative allowance — owed tax exceeds Personal Allowance	Company car benefit; prior-year debt; State Pension exceeding PA	K500 means £5,000 added to taxable income = £1,000 extra tax at basic rate per year.
NT	No Tax — employer deducts nothing	Non-UK residents; specific HMRC agreements	Extremely rare for standard UK employees. Tax debt accumulates silently if applied in error.
W1/M1	Emergency basis — non-cumulative	New starters; P45 not received	Can cause £600–£1,200 overpayment if you started work mid-year. Fix immediately via P45 or P46.
S prefix	Scottish Rate of Income Tax applies	Scottish taxpayers (S1257L, SBR, SD0, etc.)	Missing if HMRC holds a non-Scotland address. Update address in Personal Tax Account.
C prefix	Welsh Rate of Income Tax applies	Welsh taxpayers (C1257L, CBR, etc.)	Currently same rate as England — no practical effect in 2026/27 but worth having correct.

Six Reasons Your Tax Code Is Probably Wrong Right Now

HMRC updates codes reactively, not proactively — errors introduced in one year often persist for several years unless actively challenged. These are the six most common causes of incorrect codes in 2026.

1

You changed jobs and your new employer applied an emergency code

Without a P45, HMRC instructs your new employer to use W1/M1 emergency code — typically 1257L W1. This is non-cumulative: you lose credit for unused Personal Allowance from earlier in the tax year. Starting work in November on W1 can mean overpaying £600–£900 by April. Fix it by submitting your P45 or completing a Starter Checklist (P46).

2

Untaxed State Pension has reduced your code below 1257L

The full new State Pension (£11,973/year for 2026/27) is taxable but paid gross. HMRC collects the tax by shrinking your PAYE code. If you receive the full State Pension alongside wages, your code might be around 59L — not 1257L. Showing 1257L instead means you are underpaying, and HMRC will issue a January bill.

3

A company benefit-in-kind has been valued incorrectly

Company cars, private medical insurance, and interest-free loans above £10,000 are taxable via PAYE. If the benefit changed but was not reported to HMRC, your code is wrong. Check the BIK values shown in your Personal Tax Account against your P11D from your employer — any mismatch directly affects your monthly pay.

4

Marriage Allowance was registered but not applied — or not cancelled after separation

The recipient should have code 1383M (saving £252/year); the transferor should have 1131N. If either still shows 1257L, the transfer is not working. Conversely, if you separated but did not cancel the transfer, HMRC continues applying it and will claw back the underpaid tax later.

5

A prior-year tax debt has been coded out without your knowledge

HMRC can include unpaid tax from a previous year in your current code — reducing your tax-free allowance so you repay gradually through PAYE. The maximum coded out for basic-rate payers is £3,000. If your code number is unexpectedly low, check your HMRC correspondence or Personal Tax Account for a prior underpayment.

6

Your address is wrong and you are missing the Scottish or Welsh prefix

HMRC assigns S and C prefixes based on the address it holds — not where you actually live. If you moved to Scotland but HMRC has an English address, you will be taxed at UK rates rather than Scottish rates (higher above £43,662, lower on starter-rate income). Fix it by updating your address in the HMRC Personal Tax Account.

K Codes: The Tax Code That Adds to Your Taxable Income

A K code works in reverse. Instead of giving you a tax-free allowance, it adds a notional amount to your taxable income — because the tax owed on benefits or underpayments exceeds your Personal Allowance. The number after K represents the addition in hundreds: K500 means £5,000 is added to your taxable salary before the tax calculation runs.

Example — K code from State Pension + company car

Personal Allowance£12,570

State Pension (taxable, paid gross)−£11,973

Company car benefit-in-kind−£5,200

Adjustments total£17,173

Excess over PA (£17,173 − £12,570)£4,603

Tax code issuedK460

Extra tax cost at basic rate (20%)£920/year

K code cap. HMRC cannot deduct more than 50% of your pay in a single period using a K code, regardless of how large the K number is. This prevents extreme over-deduction in any one payslip, but the uncollected balance carries forward to future periods.

Emergency Tax Codes: Why W1/M1 Costs You Money and How to Fix It

An emergency code — typically 1257L W1 (weekly) or 1257L M1 (monthly) — looks almost identical to the standard code. The W1 or M1 suffix switches the calculation from cumulative to non-cumulative, with expensive consequences.

Cumulative basis (standard) — how 1257L normally works

Your employer runs a running total of all pay and all tax deducted since 6 April. Unused Personal Allowance from earlier months offsets later months' pay. Starting work in August means five months of unused allowance (£5,237) are available to reduce your first payslip's tax — effectively giving you a very low first month's tax bill.

W1/M1 emergency basis — the expensive alternative

Each pay period is treated in isolation. You receive exactly 1/12 of the annual Personal Allowance per month (£1,047) — regardless of how many months of allowance you have banked. Starting in August on W1, you receive only £1,047 tax-free in August instead of the £6,285 cumulative allowance available under a standard code. On £2,500/month salary that translates to approximately £247 of extra tax deducted that month.

How to fix an emergency code — three steps

1

Provide your P45 to your new employer immediately

A P45 carries your cumulative pay and tax to date. Once submitted, your employer switches you to a cumulative code and recovers any overpayment through your next payslip automatically.

2

Complete a Starter Checklist (formerly P46) if no P45

Tick box A (first job since 6 April), B (only job this year), or C (second job) accurately. Box C triggers BR automatically — ticking the wrong box is a very common cause of persistent incorrect codes.

3

Contact HMRC if W1/M1 persists beyond two pay periods

Phone 0300 200 3300 or update via your Personal Tax Account. HMRC will issue a corrected code to your employer. Overpayment from the W1/M1 period is refunded through payroll once the cumulative code is applied — no separate claim needed within the current tax year.

You can claim refunds for up to four tax years. Overpaid tax from incorrect codes in 2022/23, 2023/24, 2024/25, or 2025/26 can still be reclaimed. Refunds are paid to a UK bank account within 5–10 working days. The deadline for 2022/23 claims is 5 April 2027.

Decision Guide: Is Your Tax Code Correct for 2026/27?

Standard employee, one job

1257L

Should be exactly 1257L with no W1/M1 suffix. Any other number means an adjustment is in play — check your Personal Tax Account for the detail.

Scottish taxpayer

S1257L

S prefix must be present. Without it you are taxed at UK rates — potentially undertaxed above £43,662 where Scottish rates (42%) exceed English rates (40%).

Marriage Allowance recipient

1383M

£1,260 added to standard allowance. Still showing 1257L? The transfer has not been applied — you are losing £252/year.

Second job or pension

BR or D0

Second employer should receive BR (all taxed at 20%) or D0 (all at 40%). Showing 1257L on a second job means you are double-claiming the allowance and will owe tax.

Company car driver

Below 1257L

Number reduced by BIK value divided by 10. A £10,000 BIK car: code = (12,570 − 10,000) / 10 = 257, so 257L. Verify BIK value matches your P11D.

New starter mid-year

Cumulative refund

On a cumulative code, each payslip should show growing refunds of prior months' unused allowance. No refund appearing? Employer may have applied W1/M1 in error.

The fastest way to check and correct your code is through the HMRC Personal Tax Account. You can view all active codes, see each adjustment, and update your circumstances directly — triggering an automatic code update to your employer within a few weeks.

Tax Code Changes From 2025/26 to 2026/27

The standard code number did not change — the Personal Allowance remains frozen at £12,570, so 1257L stays standard. However, several circumstances trigger automatic updates that HMRC should have issued in March 2026. Check whether you received a new P2 code notice:

Circumstance	Expected change	If not updated
State Pension increased to £11,973 (April 2026)	Code reduced by ~60 units vs 2025/26	Underpaying tax on State Pension — bill due January 2027
Company car with new P11D value for 2026	Code adjusted for updated BIK value	Wrong tax deducted — over or under depending on direction of change
Marriage Allowance renewed for 2026/27	1383M / 1131N reissued	Transfer not applied — £252 benefit lost for the year
Prior-year underpayment from 2024/25 SA	Code reduced (amount coded out)	Debt accumulates — HMRC may demand direct payment
Moved to Scotland / address updated	S prefix added	Taxed at wrong rates — typically undertaxed above £26,561

Frequently Asked Questions

What is the correct tax code for 2026/27?

For most employees in England, Wales, and Northern Ireland: 1257L. Scottish taxpayers: S1257L. Welsh taxpayers: C1257L. Marriage Allowance recipient: 1383M. Marriage Allowance transferor: 1131N. Any deviation means HMRC has applied an adjustment — log in to your Personal Tax Account to see the breakdown.

How do I find my tax code?

Four places: your monthly payslip (labelled "Tax code"); your P60 (issued each April); your P45 (when leaving a job); or the HMRC Personal Tax Account at gov.uk/personal-tax-account, which shows all active codes and their components. Multiple employments and pensions each have a separate code.

How much will I overpay with the wrong code?

BR instead of 1257L on a sole income of £35,000 costs approximately £2,514/year in overpaid tax — the entire tax-free allowance is lost. W1/M1 for someone starting in November on £30,000/year can mean £800–£1,000 overpaid by April. K500 applied in error on a £40,000 salary costs £1,000/year at the basic rate. Use the Take-Home Pay Calculator with your actual code vs 1257L to quantify the gap.

How do I challenge my tax code?

Contact HMRC via your Personal Tax Account (fastest), by phone on 0300 200 3300 (8am–6pm weekdays), or by post. Explain which adjustment you believe is wrong and provide supporting evidence. HMRC will review and issue a corrected code to your employer. Current-year overpayments are recovered through payroll; prior-year overpayments are refunded directly.

What happens if I ignore a wrong code?

Overtaxed: HMRC reconciles after year end and issues a P800 refund notice — but this can take until October or November, meaning you have given HMRC an interest-free loan for up to 18 months. Undertaxed: HMRC issues a P800 underpayment notice and codes the debt out the following year or demands direct payment. Ignoring an underpayment does not make it go away.

I have two jobs — what codes should each have?

Your main job uses your Personal Allowance: 1257L (or adjusted version). Your second job should have BR if all second-job income falls in the basic-rate band, or D0 if combined income already exceeds £50,270. If the second employer applies 1257L, you are claiming the allowance twice — HMRC will calculate the underpayment and recover it the following year.