Zero-Hours Contract Emergency Tax Code: How to Fix W1/M1

Why Zero-Hours Workers Are More Likely to Get an Emergency Tax Code

Zero-hours contracts are built around flexibility. Your employer may not know whether you are a student working occasional weekends, someone with a second job, or a full-time worker with no other income. Without a P45 from a previous employer or a completed starter checklist (formerly P46), the payroll system has no basis for a proper cumulative code — so it defaults to an emergency one.

HMRC currently issues the basic rate emergency code as 1257L W1 (weekly pay) or 1257L M1 (monthly pay). The number 1257 reflects the 2026/27 personal allowance of £12,570. The letters W1 or M1 are the problem: they tell your employer to treat every pay period as if it were Week 1 or Month 1 of the tax year, ignoring any allowance you may have already used or saved up.

This matters most when your earnings are uneven — which is almost always the case on a zero-hours contract.

How W1 and M1 Tax Codes Actually Work

Under a normal cumulative code, HMRC spreads your personal allowance evenly across the tax year. If you are paid monthly, you get roughly £1,047.50 of tax-free pay each month (£12,570 divided by 12). If you earned nothing in April and May, those two months' allowances carry forward, meaning you can earn around £3,142.50 in June before paying any income tax.

Under a W1 or M1 code, that carry-forward does not happen. Each period is assessed in isolation:

• Monthly pay: you get exactly one twelfth of £12,570 = £1,047.50 tax-free per month, no more
• Weekly pay: you get exactly one fifty-second of £12,570 = £241.73 tax-free per week

If you worked no hours in April but then earned £3,000 in May under an M1 code, you would pay 20% basic-rate tax on £3,000 minus £1,047.50 = £1,952.50, so roughly £390 in tax. Under a cumulative code your April and May allowances would combine (£2,095), meaning tax of only £181 on the £905 above that figure — a difference of around £209 for that single month alone.

Step-by-Step: How to Fix Your Emergency Tax Code

Fixing a W1/M1 code is straightforward, but it requires action on your part. Your employer cannot change the code without an instruction from HMRC.

1. Check your payslip

Look for the tax code printed on your payslip. If it shows 1257L W1, 1257L M1, or simply "emergency," you are on a non-cumulative code.

2. Complete the new starter checklist if you have not already

When you started your zero-hours job, your employer should have asked you to fill in a starter checklist (the digital replacement for the old P46). There are three statements:

• Statement A — this is your only job and you have not received benefits since the start of the tax year (6 April 2026)
• Statement B — this is your only job but you received jobseeker's allowance, employment and support allowance, or incapacity benefit during the year
• Statement C — you have another job or pension

If you chose the wrong statement, or your employer never gave you the form, contact your payroll department and ask to resubmit it. This may prompt HMRC to issue a corrected code automatically.

3. Contact HMRC directly

Call 0300 200 3300 (Monday to Friday, 8am to 6pm) or log into your Personal Tax Account at gov.uk/personal-tax-account. HMRC can update your code immediately and send a P6 or P9 notice to your employer electronically. The change typically appears in the next payroll run.

4. Check your employer has received and applied the new code

After HMRC updates your code, confirm with your payroll department that they have received the notice and updated the system. Payroll teams can sometimes miss electronic notices, especially in small businesses.

5. Claim any overpayment

Once you are on the correct cumulative code, any overpaid tax from earlier in the same tax year should be refunded automatically through the payroll system as your employer recalculates on a year-to-date basis. If the tax year has already ended, HMRC will usually send a P800 tax calculation. If you do not receive one, contact HMRC or submit a claim through your Personal Tax Account.

How Much Could You Be Owed?

The overpayment depends on how uneven your earnings have been. Use a few quick figures to estimate:

For the 2026/27 tax year, the basic rate band runs from £12,571 to £50,270, taxed at 20%. National Insurance for employees is 8% on earnings between £12,570 and £50,270.

Example scenario: You work zero hours from April to July 2026 and then pick up regular shifts from August. By October you have earned £8,000 since August.

• Under a cumulative code: your allowance for April to October (7 months) is 7 × £1,047.50 = £7,332.50. All £8,000 is below that once you factor in earlier months with zero earnings... actually you would have used some allowance in August through October. The cumulative total from April to October of earnings is £8,000 against a cumulative allowance of £7,332.50, so tax owed is 20% of £667.50 = £133.50.
• Under M1 code: each month from August to October gets only £1,047.50 allowance. Assuming roughly £2,667 per month, tax each month = 20% × (£2,667 − £1,047.50) = 20% × £1,619.50 = £323.90, so total across three months = £971.70.

That is a difference of around £838 in this scenario — money you are owed back.

can help you estimate your correct liability under a cumulative code so you know what to expect when you contact HMRC.

What Happens at the End of the Tax Year

If you never resolved your emergency code during the year, HMRC will reconcile your account after 5 April 2027. They compare the tax you actually paid (via real-time information submissions from your employer) with the tax you should have paid.

If you overpaid, HMRC sends a P800 letter, usually between June and November following the end of the tax year. You can then claim the refund online (money arrives in your bank within five working days) or by cheque (takes around 45 days).

If for some reason you underpaid (for instance because you had two jobs both using the personal allowance), HMRC will issue a Simple Assessment or adjust your code in the following year to claw back the difference. This is less common with an emergency code but can happen if Statement C on your starter checklist was filled in incorrectly.

You can also check whether you are due a refund yourself at any time — do not wait for HMRC to contact you. The four-year time limit means you can claim back overpaid tax from as far back as the 2022/23 tax year until 5 April 2027.

National Insurance on Zero-Hours Contracts

It is worth separating National Insurance from income tax because the rules are different. NI is always calculated on a per-period basis — there is no cumulative concept for NI. So an emergency tax code does not affect your NI calculation; that is always correct regardless.

For 2026/27, employee NI is:

• 8% on weekly earnings between £242 and £967 (the primary threshold to upper earnings limit)
• 2% on weekly earnings above £967

If your zero-hours shifts are sporadic and you earn less than £242 in a given week, you pay no NI that week even if your annual earnings would exceed the primary threshold. This is one of the well-known features of zero-hours work that can affect your National Insurance record and ultimately your State Pension entitlement (currently £241.30 per week for 2026/27, requiring 35 qualifying years).

Tips for Avoiding Emergency Tax Codes in Future

• Always complete the starter checklist on day one — do not let your employer use the emergency code by default.
• Keep your P45 from your last job — handing it to your new employer immediately allows them to use your correct code from the first payslip.
• If you have multiple zero-hours jobs, make sure only one uses the personal allowance (Statement A). Use Statement C for additional employers, which should result in a BR or D0 code.
• Check your tax code every time you start a new engagement — zero-hours work often involves multiple short contracts, each of which could trigger an emergency code.
• Monitor your Personal Tax Account — HMRC's online service shows your current tax code and lets you flag problems without waiting on hold.

lets you compare your expected take-home under different tax code scenarios so you can spot discrepancies before they compound over months.

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This article is for information only and does not constitute financial or tax advice. Tax rules may change. Consult a qualified adviser for your specific situation.