W1 is an emergency tax code. Tax is calculated on your weekly pay only -- not your year-to-date earnings -- so you may be overpaying and should get it changed quickly.
W1 is an emergency tax code that tells your employer to calculate your income tax on a non-cumulative, week-by-week basis. Instead of spreading your Personal Allowance evenly across the tax year and adjusting for previous weeks, HMRC treats each week as if it were the very first week of the year. The result: most people on W1 overpay tax, sometimes by hundreds of pounds.
Under a normal cumulative tax code like 1257L, HMRC spreads your £12,570 Personal Allowance evenly across 52 weeks. By week 26, you have used £6,285 of your allowance. If you earned less than that in earlier weeks, the unused portion carries forward and reduces tax in later weeks. This is the cumulative system and it ensures you pay the right amount of tax by year end.
W1 throws that system out. Each week is treated as week 1. Your employer divides the annual Personal Allowance by 52 and applies exactly £241.73 per week, regardless of what happened in previous weeks. There is no catch-up. There is no carry-forward.
Worked example: £2,000 weekly salary
Under a normal cumulative code (1257L):
Under W1 (non-cumulative):
In this simple example the weekly amount is the same. The problem appears when your earnings vary week to week. If you earned nothing for weeks 1-4 (perhaps you started mid-month), a cumulative code would give you the benefit of four weeks of unused allowance in week 5. W1 does not. Those four weeks of allowance are lost, and you overpay.
W1 is not a code you choose. HMRC assigns it -- or your employer's payroll applies it -- when they do not have enough information to operate a cumulative code. The most common triggers are:
You will see W1 on your payslip alongside your tax code number. For example, 1257L W1 means you have the standard Personal Allowance but it is being applied non-cumulatively. Some payroll systems show X or NONCUM instead of W1 -- they all mean the same thing.
Almost certainly, yes -- unless your earnings are perfectly consistent every single week and you started in week 1 of the tax year (6 April).
The overpayment trap:
On a cumulative code, if you earn nothing in weeks 1-4 and then earn £2,000 in week 5, your cumulative allowance of £1,208.65 (5 x £241.73) is applied against your cumulative earnings of £2,000. You are taxed on just £791.35 for the year so far.
On W1, the same situation looks very different. Your employer ignores weeks 1-4. In week 5, you are taxed on £2,000 minus one week's allowance of £241.73 = £1,758.27. You pay tax on nearly £1,000 more than you should.
How much could you be overpaying?
The longer W1 runs, the bigger the gap can grow. Someone placed on W1 in month 4 (October) after a summer without earnings could overpay by £500-£800 or more by year end. Higher earners lose more because the excess income may push them into the 40% band on a weekly basis when a cumulative code would have kept them in the 20% band.
W1 is meant to be temporary. Here is how to get a correct cumulative code issued:
Step 1: Complete a Starter Checklist. If you have started a new job, your employer should give you an HMRC Starter Checklist (this replaced the old P46 form). You choose one of three statements:
Choose the correct statement carefully. Getting it wrong delays the fix.
Step 2: Give your P45 to your new employer. If you have one from your previous job, hand it over immediately. This gives payroll the information they need to switch to a cumulative code without waiting for HMRC.
Step 3: Contact HMRC directly. If you have completed the Starter Checklist and your code is still showing W1 after two pay periods, phone the Income Tax helpline on 0300 200 3300 (Monday to Friday, 8am to 6pm) or update your details through your HMRC Personal Tax Account.
Step 4: Wait for HMRC to issue a new code. Once HMRC has your information, they will send a new tax code to your employer electronically. This typically takes 2-3 weeks. Your employer will then switch to a cumulative basis and your next payslip should reflect the correction.
Yes. There are two ways you get the money back:
Automatic in-year refund. When your employer switches from W1 to a cumulative code, payroll recalculates your year-to-date tax. The overpayment is refunded in your next pay packet. You may see an unusually large net pay that month -- this is normal.
End-of-year reconciliation. If W1 stays on your code for the entire tax year, HMRC will reconcile after 5 April. They compare the tax you actually paid against what you should have paid on a cumulative basis. If you overpaid, HMRC sends a P800 tax calculation (usually between June and November). You can claim the refund online through your Personal Tax Account, or HMRC will post a cheque within 60 days.
Proactive claim. You do not have to wait. Contact HMRC at any time during the tax year to request a refund review. If they confirm an overpayment, they can issue a repayment directly or adjust your code so you pay less tax for the rest of the year.
All three are emergency or temporary codes, but they work differently:
| Code | Meaning | Allowance applied? | Cumulative? | Typical trigger |
|---|---|---|---|---|
| W1 | Week 1 basis | Yes (weekly fraction only) | No | New weekly-paid job without P45 |
| M1 | Month 1 basis | Yes (monthly fraction only) | No | New monthly-paid job without P45 |
| 0T | Zero allowance, all bands apply | No allowance at all | Usually yes | HMRC has no record of your circumstances |
| BR | Basic Rate, all income taxed at 20% | No allowance on this source | Yes | Second job or pension |
The key distinction: W1 and M1 give you a fraction of your Personal Allowance each period but do not carry forward unused portions. 0T gives you no allowance at all. BR applies a flat 20% to everything.
W1 is for weekly-paid employees. M1 is the identical concept for monthly-paid employees. If you see "1257L W1" on your payslip, the monthly-paid equivalent would be "1257L M1". The underlying problem and the fix are exactly the same.
1. Ignoring it because "it will sort itself out." W1 does not automatically convert to a cumulative code. HMRC needs information from you or your employer. If nobody acts, you could stay on W1 all year.
2. Not completing the Starter Checklist. Many employees skip this because they assume their P45 is enough. If the P45 is delayed or lost, the Starter Checklist is the backup that keeps you off emergency tax.
3. Choosing the wrong Starter Checklist statement. Selecting Statement A when you have a second job means your Personal Allowance is split across two employers, which can create an underpayment. Read each statement carefully.
4. Assuming W1 means you are on the wrong tax code entirely. The number part of your code (e.g. 1257) may be correct. The W1 suffix is the problem. Getting the suffix removed does not change your allowance -- it changes how that allowance is spread across the year.
5. Waiting until year end to claim a refund. You are entitled to contact HMRC at any point during the year. The sooner you act, the sooner the overpayment stops accumulating.
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