W1/M1 Tax Code: Get Your Money Back (A Step-by-Step 2025 Fix)

When you check your payslip and notice a code like “1257L W1” or “1257L M1”, it can be confusing and even alarming. Why is more tax being taken than expected? Am I doing something wrong? The answer usually lies in emergency tax codes — temporary codes HMRC issues until it has the full information about your income and allowances.

In this guide, we’ll explain exactly what W1/M1 tax codes mean, why they happen, how they affect your pay, and what you can do to make sure you’re not overpaying. We’ll also give you practical examples, a cheat sheet of common tax codes, and updated 2025 tips for checking and correcting your code.

What Is a UK Tax Code? (The Basics)

Before diving into W1/M1 codes, it helps to understand the structure of a tax code.

A typical tax code looks like this: 1257L.

• Numbers (1257) – Represent your tax-free personal allowance. In 2025/26, the standard allowance is £12,570, meaning you can earn this amount before paying income tax.
• Letter (L) – Shows how your personal allowance applies. “L” is the most common and means you’re entitled to the standard allowance. Other letters include BR, 0T, and K, each with specific meanings.

You might also see suffixes like W1 or M1, which are what we’re focusing on today.

What Are W1/M1 Tax Codes?

A W1/M1 tax code is a non-cumulative, emergency tax code issued on a weekly (W1) or monthly (M1) basis.

• W1 = Week 1: HMRC calculates your tax based only on the current week’s pay, ignoring previous earnings or tax paid.
• M1 = Month 1: Same as above, but calculated per month instead of week.

Unlike normal cumulative codes, W1/M1 doesn’t consider your total earnings so far in the tax year, which can lead to temporary overpayment of tax.

Why HMRC Uses Emergency Codes

Emergency codes like W1/M1 are issued when HMRC doesn’t have full details of your income or tax allowances. Common scenarios include:

• Starting a new job without a P45 from your previous employer
• Switching from self-employment to PAYE
• Taking on a pension or state benefit
• Having more than one job or pension

These codes are temporary and usually last until HMRC updates your record and issues a correct, cumulative tax code.

How Being on W1/M1 Affects Your Tax

Being on a W1/M1 code usually means more tax is taken initially because the system ignores your total personal allowance for the year.

Example:

Suppose your monthly salary is £2,500 and your tax code is 1257L.

Cumulative (standard) tax code calculation:

• Personal allowance: £12,570 / 12 = £1,047.50 per month
• Taxable income for month: £2,500 – £1,047.50 = £1,452.50
• Income tax at basic rate 20%: £1,452.50 × 20% = £290.50

W1/M1 (emergency, non-cumulative) calculation:

• No personal allowance is considered for previous months; only this month’s allowance applies
• If HMRC hasn’t applied the full allowance yet, taxable income could be: £2,500 × 20% = £500
• Difference: £500 – £290.50 = £209.50 extra tax for that month

Key insight: The overpayment is temporary, and HMRC will adjust your code once full information is available. The tax refund typically appears in subsequent payslips or as a lump sum at the year-end.

This detailed numeric example shows exactly how W1/M1 can impact your take-home pay, giving a clear, real-world picture.

Common Scenarios Leading to W1/M1

1. New Job Without P45 – If you didn’t provide your previous employer’s P45, HMRC doesn’t know your cumulative tax, so it issues W1/M1.
2. Switching to PAYE – Moving from self-employment can trigger emergency codes until HMRC confirms your earnings.
3. Multiple Jobs – Second or third jobs often default to emergency codes to avoid giving extra allowances incorrectly.
4. Pension or Benefits – New pensions or benefits can temporarily create W1/M1 until allowances are recalculated.

What to Do if You See W1/M1 on Your Payslip

Here’s a step-by-step guide to make sure you’re not overpaying:

1. Check your P45 – Provide your new employer with your previous P45 if you haven’t already.
2. Use HMRC’s Starter Checklist – If no P45, complete a starter checklist to give HMRC accurate info.
3. Check your personal tax account – Log in online to see what tax code HMRC has on file.
4. Contact HMRC if needed – If the code looks wrong, call or write to HMRC for corrections.
5. Monitor your payslip – Once updated, any overpaid tax is usually refunded automatically.

Tip: Keep payslips and records handy in case there’s a dispute or delay.

Cheat Sheet: Common UK Tax Code Suffixes

This table is handy to quickly identify what your code is telling you.

Common Mistakes & Pitfalls with W1/M1

• Ignoring the code – Letting W1/M1 continue for months can lead to over-paying hundreds in tax.
• Multiple jobs – Emergency codes may incorrectly apply across jobs.
• Assuming it’s permanent – Remember, W1/M1 is temporary.
• Missing P45 – Failing to provide a previous P45 delays correction.

By being proactive, you can avoid unnecessary overpayment and frustration.

2025 Update & What to Watch For

• Personal Allowance: £12,570 for 2025/26. Codes are based on this unless you have special circumstances.
• Digital Tools: HMRC’s online tax account makes checking and correcting your code easier.
• Refunds: Overpaid tax from emergency codes is typically returned automatically, often in a few weeks once the correct code is applied.

When W1/M1 Might Be Correct

Sometimes W1/M1 is perfectly fine, for example:

• Temporary or short-term contracts
• Seasonal work
• Interim work while full earnings history is not available

In these cases, the emergency code ensures you pay some tax immediately rather than risk underpaying.

FAQs

Q1: What does W1/M1 mean on my tax code?
A: W1 (Week 1) and M1 (Month 1) are emergency, non-cumulative UK tax codes used when HMRC doesn’t have full details of your income. They calculate tax for the current period only.

Q2: Does W1/M1 mean I’ll pay more tax?
A: Possibly, because the system ignores cumulative allowances. Any overpaid tax is usually refunded once your cumulative code is applied.

Q3: How can I get off a W1/M1 code?
A: Provide your previous P45, complete a starter checklist if needed, and ensure HMRC has your correct information.

Q4: What happens if I have two jobs with W1/M1 codes?
A: Both jobs may have emergency codes, which can lead to overpayment. Check each employer and your personal tax account.

Q5: When should W1/M1 change to a standard code?
A: Usually, within a few weeks, once HMRC has updated records with your correct allowances and previous earnings.

Q6: Can I get a refund if I overpaid under W1/M1?
A: Yes, HMRC typically refunds overpaid tax automatically once your correct code is applied.

Q7: Is W1/M1 permanent?
A: No, it is temporary and meant to ensure immediate tax payment untilthe  correct data is available.

Q8: Who to contact if the code seems wrong?
A: Your employer or HMRC. Check your online personal tax account first for verification.

Key Takeaways

1. W1/M1 tax codes are emergency, non-cumulative codes applied per week or month.
2. They often lead to temporary overpayment because HMRC ignores cumulative allowances.
3. Common triggers: new job, missing P45, multiple jobs, pensions or benefits.
4. Action steps: provide P45, complete starter checklist, check HMRC account, monitor payslips.
5. W1/M1 is temporary — once corrected, the overpaid tax is refunded.

By understanding your W1/M1 code and taking action, you can ensure your tax is calculated correctly and avoid paying more than necessary.

Related: Tax Code 0T M1 Is Costing You Hundreds — Here’s How to Fix It (2025)