Emergency Tax Calculator UK
Estimate Income Tax and National Insurance for a weekly or monthly payslip using the gross pay and code you select. The result is an illustration, not confirmation that HMRC owes you a refund.
Emergency Tax & W1/M1 Checker (2026/27)
Estimate deductions on a payslip using an emergency tax code and see what to check if the amount looks wrong.
Emergency tax splits your allowance into 12 equal monthly chunks (£1,047.50).
Look at your payslip code for "W1", "M1", or "X".
The comparison shows how the selected code differs from a 1257L W1/M1 estimate for this pay period. It cannot confirm whether you have overpaid for the tax year or whether HMRC will issue a repayment.
How Emergency Tax Works & Why It Deducts Extra
What the 'W1' or 'M1' Symbol Means
Standard UK PAYE is cumulative: unused personal allowance from months you were unemployed rolls forward and cuts your tax bill.
Under an emergency code like 1257L W1 or 1257L M1, your payroll software cannot see your previous earnings. It treats this payslip completely in isolation, giving you only £1,047.50 of tax-free allowance and applying full tax on everything else.
What to check if you think you overpaid
Once HMRC reviews your details, it may update the tax code sent to your employer. Payroll then uses that code for later pay runs; whether this corrects an overpayment, and when, depends on your full pay and tax record for the year. Check your HMRC Personal Tax Account or contact payroll if you think you have paid too much tax.
You can review your employment and income details in your HMRC Personal Tax Account or app and follow HMRC’s current instructions. Timing and any repayment depend on your full tax-year position.
Claim Back Overpaid Emergency Tax (HMRC Rebate)
If your payslip shows 1257L W1, 1257L M1, or 0T, your employer deducted Income Tax on a non-cumulative basis. This ignores your accumulated tax-free Personal Allowance from earlier months or employment gaps. You could be owed hundreds of pounds in an HMRC P800 rebate.
If your code looks wrong, compare it with your HMRC Personal Tax Account and ask payroll how it was applied. Read our emergency tax-code guide, tax-code guide, or the guide to PAYE on a payslip.
UK Emergency Tax Codes Decoded (2026/27)
Emergency tax codes are statutory holding mechanisms enforced by HMRC under the Income Tax (Pay As You Earn) Regulations 2003 (SI 2003/2682) and the Income Tax (Earnings and Pensions) Act 2003 (ITEPA 2003). They ensure an employee pays tax even when full employment records or historic P45 certificates are unavailable at the time payroll runs.
You receive 1/52nd (£241.73/wk) or 1/12th (£1,047.50/mo) of the tax-free allowance for that specific pay cycle, but cannot claim unused allowances from earlier in the tax year.
Assumes £0 tax-free allowance. Every pound of earnings is taxed at 20% (up to £3,141/mo), 40% (between £3,141 and £10,428/mo), and 45% on the excess.
Deducts a flat 20% Income Tax from every penny earned with zero personal allowance. Frequently applied to second jobs or missing starter checklists.
Emergency Code (W1/M1) vs Normal Cumulative PAYE
The fundamental difference between emergency tax codes and standard PAYE is cumulation. Under normal cumulative PAYE, your personal tax allowance accumulates each month across the UK tax year (6 April to 5 April). Under an emergency W1 (Week 1) or M1 (Month 1) code, each pay period is assessed in isolation:
| Feature & Payroll Rule | Normal Cumulative PAYE (1257L) | Emergency Code (1257L W1 / M1 / X) |
|---|---|---|
| Allowance Allocation | Accumulates: Receives £1,047.50 × Month number. Month 4 receives £4,190.00 tax-free allowance. | Fixed per period: Strictly limited to 1/12th (£1,047.50/mo) or 1/52nd (£241.73/wk). Past unused allowance is disregarded. |
| Treatment of Mid-Year Gaps | Unused allowance from months without earnings rolls forward, often resulting in £0 Income Tax on your first payslip. | Disregards gap months completely. Full tax is deducted as if you had worked continuously, causing temporary overdeduction. |
| Higher Rate Thresholds | 40% Higher Rate band applies only after cumulative taxable pay exceeds £4,189.17 × Month number. | 40% Higher Rate band triggers as soon as that single month's earnings exceed £4,189.17. |
| How Repayments Occur | Immediate in payroll: Automatic running reconciliation between gross earnings and total tax paid to date. | Automatic upon code update: When HMRC issues your cumulative code (e.g. 1257L), payroll software automatically repays overpaid tax in that next payslip. |
| End-of-Year P800 Rebate | Annual tax deducted matches exact statutory liability. P800 rarely needed. | If employment ends or code was not corrected, HMRC sends a P800 tax calculation after 5 April with a direct refund. |
| Statutory Legislation | SI 2003/2682 Reg 42 & Reg 43 | SI 2003/2682 Reg 44 (Week 1/Month 1) |
Numerical Example: Starting Work in Month 4 (July) Earning £3,500/Month
The table below contrasts the tax deductions for an employee earning £3,500 gross per month (£42,000/yr) who joins a new company in Month 4 (July) having had zero earnings in Months 1 to 3:
| Payslip Parameter | Standard Cumulative (1257L) | Emergency Code (1257L M1) | Emergency Code (0T) |
|---|---|---|---|
| Month 4 Gross Pay | £3,500 | £3,500 | £3,500 |
| Available Allowance | £4,190 (4 months accumulated) | £1,048 (1 month only) | £0 (Zero allowance) |
| Taxable Pay in Month 4 | £0 (Fully covered) | £2,452 | £3,500 |
| Income Tax Deducted | £0.00 | -£490.50 (20%) | -£771.60 (20% & 40%) |
| Class 1 National Insurance | -£196.16 | -£196.16 | -£196.16 |
| Net Monthly Take-Home Pay | £3,303.84 | £2,813.34 | £2,532.24 |
| Difference vs 1257L estimate | £0 (Exact) | £490.50 estimated difference | £771.60 estimated difference |
What to check if your emergency-tax code looks wrong
A W1 or M1 code can affect the timing of Income Tax deductions, but it does not prove you have overpaid for the tax year. Check your full pay and tax record in your HMRC Personal Tax Account and ask payroll if you think the code is incorrect. HMRC can update a code when needed, and the final tax position depends on your circumstances.
Emergency Tax & Payslip Checks (2026/27)
Learn how emergency tax codes work and what to check if your payslip deductions look unexpected.
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Visit Blog HubAnnual employment estimate
Use the stated tax code and gross pay to compare employment deductions in England, Wales and Northern Ireland. Monthly, weekly and daily figures divide an annual result; they do not reconstruct cumulative payroll.
The main pension input models salary sacrifice. Irregular pay, benefits in kind, multiple employments and a different pension method can change a real payslip. Use the Scottish tool for Scottish employment income.
Check the rules for your circumstances
Official guidance supports the rules described here; it does not approve or certify this calculator. Confirm dates, eligibility and unsupported cases before making a financial decision.