Emergency tax codes explained

What an 'emergency' tax code actually is

An emergency tax code is what your employer uses when they don't yet have enough information to apply your correct, settled code — typically when you start a new job without a P45 from your previous employer, or haven't completed a new starter checklist. Rather than guessing, payroll software applies a standard temporary code so you're not left completely untaxed while things get sorted.

The genuinely 'emergency' code HMRC refers to is the standard 1257L code applied on a W1, M1 or X basis — meaning you still get the normal Personal Allowance, just worked out fresh each pay period rather than cumulatively across the year so far.

Cumulative vs non-cumulative (W1/M1) codes

Most tax codes work cumulatively: your allowance builds up through the year, and each payslip looks at your total pay and total allowance since 6 April, so it self-corrects as the year goes on. A W1 (week 1) or M1 (month 1) code instead resets every pay period — you only get that period's slice of the annual allowance, with no credit for any unused allowance from earlier in the tax year.

This mainly bites people who start a job partway through the tax year with significant unused allowance sitting unclaimed from the months before they started earning. On a cumulative code, that unused allowance offsets tax due now; on an M1 or W1 code, it's temporarily lost — though it's usually sorted out once HMRC issues the correct code or through a tax return.

A worked example: starting in month 7 on M1 vs cumulative

Imagine starting a new job in October (month 7 of the tax year) on £3,000 a month, with no other income earlier in the year. Under a cumulative 1257L code, your allowance to date is 7/12 of £12,570, or £7,332.50 — comfortably more than the £3,000 you've actually earned so far, so no tax is due yet. Under an M1 code, you only get 1/12 of the allowance for that single month — £1,047.50 — against that month's £3,000, leaving £1,952.50 taxable at 20%, or £390.50 of tax. Same income, same eventual correct position for the year, but a very different amount of tax taken in that first payslip.

Other common temporary codes

• 0T — no Personal Allowance at all, every pound taxed through the normal bands; often used when an employer has no starter information whatsoever.
• BR — everything taxed at 20% with no allowance; common for a genuine second job, but sometimes wrongly applied to a main job when the starter checklist is filled in incorrectly.
• D0 / D1 — everything taxed at 40% or 45%; usually correct only for a second source of income once you're already a higher or additional-rate taxpayer.
• 1257L W1/M1/X — the standard emergency code: full allowance, but applied non-cumulatively until HMRC or payroll switches you back to a normal cumulative basis.

How to get it fixed

The fastest fix is usually giving your new employer the information they're missing: your P45 from a previous job if you have one, or a completed starter checklist stating clearly whether this is your only or main job. Once HMRC has the full picture, they issue a correct tax code directly to your employer, and most payroll systems switch back to a cumulative basis automatically, which can trigger a welcome one-off refund through your pay if you were overtaxed on an M1 or W1 basis earlier in the year.

If the tax year ends before it's corrected, any overpayment is usually caught either automatically (HMRC sometimes issues a P800 calculation after the year ends) or by contacting HMRC directly — it's rarely lost for good, just delayed.

Common mistakes

• Assuming an emergency code means something has gone wrong — it's often just a temporary placeholder while your employer waits for the right paperwork, not an error in itself.
• Not providing a P45 or completing a starter checklist promptly — this is usually the single fastest way to get off an emergency code.
• Assuming a BR or 0T code on a genuine second job is a mistake — it can be entirely correct if your Personal Allowance has already been allocated to your main job.
• Not checking whether an M1/W1 code has corrected itself later in the year — some employers only switch to cumulative once instructed, so it's worth checking a few payslips later if the code hasn't changed.
• Giving up on reclaiming overpaid tax from an emergency code — it's usually recoverable either automatically at year-end or by contacting HMRC directly.