The Week 1 tax basis is a non-cumulative way of running PAYE: your employer works out income tax on each paycheck on its own, ignoring what you’ve earned or paid so far in the tax year. You’ll see it on your payslip as a W1, M1, or X suffix after your tax code (for example, 1257L W1). It usually appears when HMRC doesn’t yet have the full picture of your income, and it tends to produce slightly higher deductions than a standard cumulative code because unused personal allowance from earlier weeks or months can’t be applied.
How the Non-Cumulative Method Changes Your Pay
Under normal cumulative PAYE, your employer tracks total earnings and total tax paid from 6 April onward. Each payday the payroll software recalculates whether you’ve paid too much or too little across the year to date and adjusts the next deduction. That’s why a quiet month or a period of leave can produce a small refund built into your next payslip.
The Week 1 basis switches that off. Regulation 26 of the Income Tax (Pay As You Earn) Regulations 2003 directs employers to apply the non-cumulative basis when HMRC instructs them to or when the regulations themselves require it, and while it applies the cumulative method stops entirely.1Legislation.gov.uk. The Income Tax (Pay As You Earn) Regulations 2003 – Regulation 26 Each week or month is treated as if it were the only pay period in the year.
The practical effect is in how your personal allowance is sliced. The standard allowance is currently £12,570. A weekly-paid employee on the Week 1 basis gets that divided by 52, giving roughly £241.73 of tax-free pay each week. A monthly-paid employee on the Month 1 basis gets £12,570 divided by 12, or about £1,047.50 tax-free each month. Earnings above that slice are taxed at the basic rate of 20%, with higher-rate bands of 40% and 45% applying if a single period’s pay is large enough.2GOV.UK. Income Tax Rates and Personal Allowances
Unused allowance doesn’t carry forward. Earn nothing one week and £1,000 the next, and the cumulative system would let both weeks’ allowance apply to that £1,000. The Week 1 basis won’t. The second paycheck gets only its own £241.73 slice, and the first week’s share is lost. This is where most people feel the difference: irregular hours or fluctuating shifts produce higher deductions than expected because the system can’t smooth things out across pay periods.
What Puts You on a Week 1 Code
Starting a new job is the most common trigger. If your new employer doesn’t have your previous income and tax details, they’ll operate an emergency code with a W1 or M1 suffix until HMRC catches up.3GOV.UK. Tax Codes – Emergency Tax Codes Handing over your P45 from your old employer speeds this up because it gives your new payroll department the year-to-date figures. Without a P45, you fill in a starter checklist, which your employer uses to set an initial code and register you with HMRC.4GOV.UK. Starter Checklist if Youre Starting a New Job
New taxable benefits can also trigger it. If you start receiving a company car, private medical insurance, or the State Pension partway through the year, HMRC may place you on an emergency code so the extra tax is collected straight away rather than building into a shortfall.3GOV.UK. Tax Codes – Emergency Tax Codes When benefits are the cause, the emergency code usually stays until the end of the tax year, and HMRC moves you onto a standard cumulative code from the following 6 April.
Multiple income sources are another frequent reason. When you hold two jobs at once, HMRC needs the personal allowance applied only once, so the secondary employer often runs a non-cumulative code to avoid giving you a double allowance and a large underpayment at year end.
How Long It Lasts
For most new starters the emergency code is temporary. HMRC normally updates the code once details from both your new and previous employers have come through, which can take up to 35 days from your start date.3GOV.UK. Tax Codes – Emergency Tax Codes Once the updated code arrives, your employer switches to the cumulative basis and payroll goes back to accounting for the full year-to-date picture.
Sometimes it lingers. If you haven’t paid enough tax under the emergency code, HMRC may keep you on it until the shortfall is recovered. And emergency codes triggered by new company benefits or the State Pension typically stay in place until the following 6 April.3GOV.UK. Tax Codes – Emergency Tax Codes
How to Check and Correct Your Code
If you think your Week 1 code is wrong or overdue for an update, the quickest route is the “Check your Income Tax” service on GOV.UK. Sign in through your Personal Tax Account and review your employment details, pension information, estimated income, and any company benefits, updating anything that’s missing or incorrect.5GOV.UK. If You Think Your Tax Code Is Wrong The same service lets you report changes in circumstances that affect your code, such as a new job or a shift in benefits.6GOV.UK. Check Your Income Tax for the Current Year
If you can’t use the online service, contact HMRC by phone. HMRC advises waiting at least 35 days after starting a new job before calling, because the system often sorts itself out once employer records are exchanged.5GOV.UK. If You Think Your Tax Code Is Wrong Have your employer’s PAYE reference, the tax code from a recent payslip, and your estimated income for the rest of the year to hand, along with your P45 or most recent P60 if you have them.7GOV.UK. P60
Once HMRC processes a change, they issue a new tax code to you on a P2 coding notice and to your employer, within 15 working days of the update.5GOV.UK. If You Think Your Tax Code Is Wrong Monthly employees should see the new code on their next payslip or the one after. Weekly employees should expect it by their third payslip after the code is issued. If it doesn’t appear, check with your employer that they received the notification. When the cumulative code takes effect, payroll recalculates your tax for the whole year to date, and any overpayment from the Week 1 period should come back to you through your pay over the following periods.
Year-End Reconciliation if It Wasn’t Fixed in Time
Even if your code isn’t corrected during the year, HMRC runs a reconciliation after the tax year ends on 5 April, comparing the tax you paid through PAYE against what you actually owed on your full-year income. If the two don’t match, HMRC sends a tax calculation letter called a P800.8GOV.UK. Tax Overpayments and Underpayments
P800 letters go out between June and March of the following year, so the timeline isn’t fast.8GOV.UK. Tax Overpayments and Underpayments If you overpaid, the letter explains how to claim your refund. If you underpaid, it tells you what you owe and how to pay. Anyone who spent a significant part of the year on a Week 1 basis with fluctuating earnings is more likely to receive one, because the non-cumulative method rarely lands on exactly the right annual figure.
If you think you’ve overpaid and haven’t had a P800, use the “Check your Income Tax” service or contact HMRC to ask for a review. Silence from HMRC isn’t the same as a clean balance, particularly after several months on a non-cumulative code.