Emergency Tax Codes UK Explained: Why You're Paying Too Much Tax

Quick answer: An emergency tax code is a temporary code (usually 1257L W1 or 1257L M1) used when HMRC doesn’t have enough information about your income. It taxes each pay period independently without considering your annual Personal Allowance, often causing overpayment. You usually get put on an emergency code when starting a new job. Once HMRC has your details, your employer will receive the correct code automatically. Source: GOV.UK — Emergency tax codes.

Emergency Tax Codes UK Explained

If your payslip shows a tax code ending in W1 (week 1), M1 (month 1), or X, you’re on an emergency tax code. This means you could be paying too much tax. This guide explains why and how to fix it.

What Is an Emergency Tax Code?

An emergency tax code is used when HMRC doesn’t have complete information about your earnings. The most common emergency code for 2025/26 is 1257L W1 or 1257L M1:

W1 = Week 1 (for weekly-paid employees)
M1 = Month 1 (for monthly-paid employees)
X = used in some payroll systems instead of W1/M1

These suffixes mean your tax is calculated on a non-cumulative basis — each pay period is taxed independently without reference to what you’ve earned earlier in the tax year. This often results in overpaying tax because your Personal Allowance isn’t properly applied.

Read our how to read a payslip guide to understand where your tax code appears.

Why Are You on an Emergency Tax Code?

Common reasons:

Situation	What Happens
Starting a new job	Employer doesn’t have your P45 yet
No P45 provided	HMRC has no record of your previous earnings
Multiple jobs	HMRC can’t determine your primary employment
Tax code change	HMRC is updating your code mid-year
New to the UK	No UK tax history on file
Returning to work	After a gap, HMRC needs updated information

How Emergency Tax Works (Example)

Normal (cumulative) tax code 1257L:

You earn £2,500/month
Monthly Personal Allowance: £1,047.50 (£12,570 / 12)
Taxable: £1,452.50 at 20% = £290.50 tax/month

Emergency code 1257L M1 (same salary):

Same calculation per month
BUT if you started in Month 7 (October), you’d have been taxed as if earning £2,500 from Month 1
You’d owe £2,034 in tax in Month 7 instead of £290 — because prior months’ untaxed allowance is ignored
This overpayment is recovered when your correct code arrives

Result: You could temporarily overpay hundreds of pounds until HMRC corrects your code.

How to Get Off an Emergency Tax Code

Method 1: Wait (Automatic)

In most cases, HMRC will automatically update your code once they receive your employment details (usually within 4-8 weeks). Your employer will then apply the correct code and any overpaid tax is refunded via your payslip.

Method 2: Provide Your P45

Give your P45 from your previous employer to your new employer’s payroll department. They’ll use it to set the correct tax code immediately.

Method 3: Update HMRC Online

Sign in to your personal tax account on GOV.UK
Go to “Check your Income Tax” → “Tell HMRC about a change”
Update your employment details
HMRC will issue a new tax code to your employer

Method 4: Call HMRC

If the online method doesn’t work, call HMRC on 0300 200 3300. Have your National Insurance number ready.

Source: GOV.UK — Check your Income Tax.

Will You Get a Tax Refund?

If you’ve overpaid tax on an emergency code, you’ll usually get it back automatically:

During the tax year: Your employer refunds the overpayment when your correct code is applied
After the tax year ends: HMRC sends a calculation (P800) and arranges a refund

You can also claim a refund yourself at GOV.UK — Claim a tax refund.

Use our income tax calculator to check what you should be paying.

Frequently Asked Questions

“What does W1 or M1 mean on my tax code?” | “W1 (Week 1) and M1 (Month 1) mean you’re on an emergency tax code. Your tax is calculated for each pay period without considering what you’ve earned earlier in the tax year, which can cause overpayment. HMRC will usually correct this automatically within 4-8 weeks.” “How do I get off an emergency tax code?” | “Give your P45 to your new employer, or update your employment details in your GOV.UK personal tax account. HMRC will then issue the correct code to your employer, usually within 4-8 weeks. Any overpaid tax is refunded automatically.” “Will I get my overpaid tax back?” | “Yes. When your correct tax code is applied, any overpaid tax is refunded through your payslip during the tax year. If the year has ended, HMRC will send a P800 calculation and arrange a refund.” “What is the emergency tax code for 2025/26?” | “The standard emergency tax code for 2025/26 is 1257L W1 or 1257L M1. The 1257L part gives you the standard £12,570 Personal Allowance, but the W1/M1 suffix means it’s calculated non-cumulatively.”

Sources and Further Reading

Disclaimer: Tax codes are specific to your individual circumstances. Always check your code on GOV.UK or contact HMRC directly if you believe you’re on the wrong code.