Wk1 Tax Code: Why You’re On It and How to Fix It

The WK1 tax code (also written W1, or M1 if you’re paid monthly) is an emergency PAYE code that tells your employer to work out your tax on the current pay period alone, ignoring anything you’ve earned or paid earlier in the tax year.¹ HMRC applies it when it doesn’t have enough information to work out your correct position, most often when you start a new job without a P45. It’s temporary, and you fix it by giving HMRC the missing income details so it can issue a normal cumulative code.

How WK1 Changes Your Pay

The WK1 or M1 suffix sits at the end of an otherwise ordinary code, like 1257L WK1. The number and letter still represent your tax-free allowance. What changes is how your employer applies it. Under the standard cumulative method, your employer adds up all pay and tax from 6 April onward, so any unused allowance from earlier low-earning or non-earning weeks rolls forward and reduces later tax bills. The WK1 method throws that out. Each period gets a flat slice of your annual allowance, and previous periods are invisible.²

Weekly-paid workers get 1/52 of the personal allowance each week; monthly-paid workers get 1/12 each month. Everything above that slice is taxed at the normal rates for that period’s income.²³

The overpayment shows up most obviously when you didn’t work at the start of the tax year. If you were out of work from April through August and started a job in September on a WK1 code, five months of unused personal allowance would normally roll forward and cut your September tax. Under WK1, those months don’t exist as far as payroll is concerned. You get only the current period’s slice, and everything above it is taxed as though the year had just begun.

HMRC’s stated aim with this basis is to stop employers from making heavy deductions or issuing refunds during periods of uncertainty.² The trade-off is a code that’s rarely exactly right but also never dangerously wrong. If you’d already earned a lot at another job earlier in the year, WK1 keeps a new employer from applying too much allowance and leaving you with a surprise bill later.

Why You’ve Been Put on It

The usual trigger is starting a new job without giving your new employer a P45 from the previous one. Without that record, HMRC has no way to know what you’ve already earned and paid this tax year, so it defaults to the non-cumulative basis.⁴

When there’s no P45, employers use HMRC’s starter checklist. You pick one of three statements:⁵

Statement A: this is your first job since 6 April and you haven’t received Jobseeker’s Allowance, Employment and Support Allowance, or Incapacity Benefit. You get the personal allowance on a cumulative basis.
Statement B: you’ve had another job this tax year but don’t have a P45, or you’ve received certain state benefits. You get the personal allowance on a week 1/month 1 basis. This is the WK1 code.
Statement C: you have another job or a pension alongside this one. You get the BR code, taxing all income from this job at the basic rate with no personal allowance.

Statement B is where most people land when a P45 has been lost or a previous employer is slow to issue one. Other triggers include starting to receive company benefits such as a company car or private medical insurance partway through the year, or returning to work after a long break where HMRC has no recent income data on you.⁴

What to Gather Before You Contact HMRC

A few items in front of you will save time on the call or online form. The most important is your P45. It has three parts: Part 1A is yours to keep, and Parts 2 and 3 go to your new employer. If they’re still in a drawer at home, hand them to payroll straight away. If your previous employer never issued one, ask them for it.⁴

Beyond the P45, have these to hand:

Your National Insurance number, which you can find on a P60, payslip, or letters about benefits.⁶
Your employer’s PAYE reference, in the format 123/AB456, shown on your payslip or on letters your employer has had from HMRC.⁷
Your most recent payslip, to confirm the exact code being used.
An estimate of your total income for the tax year, so HMRC can build a correct cumulative code.

How to Get the Code Changed

The quickest route is the HMRC Personal Tax Account, where you can check your current code, update income details, and report changes.⁸ You can do the same through the HMRC app. Either way you’ll need to verify your identity, which may involve matching a photo of your face to your passport or driving licence, or answering security questions drawn from information HMRC already holds such as a recent P60 or payslip.⁹

If you’d rather speak to someone, the income tax helpline is 0300 200 3300. Expect automated identity checks before you reach an agent. Whichever route you use, the goal is to give HMRC enough information to issue a correct cumulative code.

Your employer can’t change the code on their own. They have to wait for HMRC to send them a P6 notice with the new code; you’ll get a P2 notice confirming the change. HMRC aims to do this within 15 working days.¹⁰

When Your Pay Returns to Normal

Once payroll receives the new cumulative code, timing depends on how often you’re paid. Monthly, the corrected code should appear on your next payslip or the one after. Weekly, expect it by your third payslip.¹⁰ If it hasn’t landed by then, check with payroll to confirm the P6 arrived.

If HMRC has your full income details for the year when they update the code, they’ll work out the difference between what you’ve paid and what you owe and tell your employer to refund the overpayment through your pay. It comes through automatically, usually on the first payslip under the new code, and you don’t need to file a separate claim.¹¹ The refund can be sizeable if the WK1 code ran for several months after a period of no income, because the cumulative code effectively catches up on all the unused allowance.

If the Tax Year Ends Before It’s Fixed

The 5 April cut-off doesn’t wipe out the discrepancy. After the year closes, HMRC compares the tax you actually paid with what you should have paid on your full-year income. If the two don’t match, they send either a P800 tax calculation or a Simple Assessment.¹² Being on the wrong tax code is one of the reasons HMRC lists for issuing a P800.

These letters are sent between June and March of the following tax year. If you use a Personal Tax Account, the calculation may appear online as early as June; by post, expect it closer to autumn. A P800 showing an overpayment explains how to claim your refund. One showing an underpayment explains what you owe.¹² Sorting the code out during the tax year is almost always easier than waiting for this reconciliation.

Deadline to Reclaim Overpaid Tax

You have four years from the end of the tax year in which you overpaid to claim it back. After that, HMRC treats the year as settled and the money is gone. For the 2025–26 tax year, ending 5 April 2026, the deadline is 5 April 2030. If you think a WK1 code caused you to overpay in an earlier year and you never received a P800, check your Personal Tax Account or contact HMRC before that four-year window closes.¹³

1
GOV.UK. Understanding Your Employees Tax Codes
2
HMRC Internal Manual. PAYE Manual – Coding: Codes: How They Are Used and Calculated: Ways an Employer Can Apply a Tax Code
3
GOV.UK. Income Tax Rates and Personal Allowances
4
GOV.UK. Tax Codes – Emergency Tax Codes
5
HM Revenue and Customs. Starter Checklist
6
GOV.UK. Find Your National Insurance Number
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HMRC Patterns for Services. Employer PAYE Reference
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GOV.UK. Check Your Income Tax for the Current Year
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GOV.UK. Sign Up for Making Tax Digital for Income Tax
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GOV.UK. Tax Codes – If You Think Your Tax Code Is Wrong
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GOV.UK. Tax Codes – If You Have Paid Too Much or Too Little Tax
12
GOV.UK. Tax Overpayments and Underpayments
13
GOV.UK. HMRC Internal Manual – Compliance Handbook – What Are the New Time Limits – Introduction