You are on an emergency tax code if your code ends in W1, M1, X or NONCUM. Your tax is then worked out on what you are paid in that week or month alone, as though you were paid the same in every period of the year, instead of on your total pay since 6 April. You still get the Personal Allowance, but only one period of it at a time, which is why starting a job late in the tax year can cost you until the code changes.
What the letters at the end mean
An emergency code is an ordinary code with a marking after it, and the markings say the same thing in different payroll dialects. W1 is weekly pay taxed week by week. M1 is monthly pay taxed month by month. X covers pay dates that vary. NONCUM is the wording some payroll software prints instead.
If your code does not end in one of those, you are not on an emergency code, whatever else it says. 1257L is the ordinary code; 1257L M1 is the emergency version of it. HMRC's P9X confirms that 1257L is the emergency code for 2026/27.
Cumulative against non-cumulative
Normally PAYE is cumulative, which means it counts the whole year each time. Each pay day your employer looks at everything you have earned and paid in tax since 6 April, works out the tax due on the year so far, and deducts the difference. Allowance you did not use in earlier months comes back to you automatically.
An emergency code switches that off. Your employer treats each pay period as if it were repeated all year, and gives you one period's worth of allowance and no more. On a monthly payroll that is £1,048.26; on a weekly one, £241.92. Everything you earned before that period is invisible to the sum.
The result depends on when you started. Starting in April costs you almost nothing, because one period's allowance is what you were entitled to anyway. Starting in October means six months of unused allowance is sitting there, unclaimed, until a cumulative code arrives. And an irregular payment, such as a bonus in your first month, is taxed as though you will receive it every month, which can push part of it into a higher rate it never deserved.
Emergency codes can also take too little. Leave a well-paid job in June and start a lower-paid one in July, and month-by-month taxation can undercharge you. That tax is not written off. A cumulative code counts the whole year again, so the shortfall comes back into the sum when one arrives. If the tax year ends before that happens, HMRC reconciles it after 5 April and writes to you.
Why you have one
GOV.UK gives three common reasons. You started a job without giving your new employer a P45, so they have no record of your pay and tax so far. Your employer has had no code notice from HMRC yet. Or you have started getting company benefits or the State Pension, which HMRC calls normal and which is there to collect the right tax for the rest of the year.
You can help it along. Hand over the P45 your last employer gave you when you left, or fill in the starter checklist if you do not have one.
How long it lasts
After a new job, HMRC usually updates the code once it has the details from your old and new employers. That can take up to 35 days from the day you start. If you have been there longer than that and the code has not changed, that is the point to check your details rather than wait. GOV.UK adds one condition: if you have not paid enough tax, you stay on the emergency code until you have paid the right tax for the year.
If the code came from a company benefit or the State Pension, it is meant to last. GOV.UK says you will have an emergency code until the end of the tax year, and a non-emergency code in the new one. That is the code working as designed rather than a fault to chase.
Either way, the marking does not travel. Employers are told not to carry a week 1 or month 1 marking into the new tax year, so the following year starts cumulatively.
Getting the overpayment back
Once a cumulative code reaches your employer, the payroll works out your tax for the year to date, sees what you have already paid, and refunds the difference through your pay. Paid monthly, that is typically your next payslip or the one after; paid weekly, typically your third.
If the tax year has ended first, HMRC checks your record after 5 April and writes to you about a refund. Either route depends on HMRC holding your income details for the year, which is why the fastest thing you can do is make sure it has them.
Is an emergency code the same as BR or 0T?
No. An emergency code still gives you the standard allowance, one period at a time. BR and 0T give you no allowance at all. Those two can be operated on a week 1 or month 1 basis as well, which is why an overpayment under BR or 0T is the larger one and why it is worth reading the whole code rather than only the ending.
How do I know how much I have overpaid?
Compare the tax on your payslip with the tax the year should produce for your salary. On £30,000 with the standard code, a full year works out at £2,093.34 a month in take-home pay; the difference between that and what has actually reached your bank is roughly what you are waiting on. It is only an approximation until the year has run its course, because a cumulative code settles the year rather than any one month.
Do I need to phone HMRC?
Not usually. Most emergency codes clear themselves once your employer reports your first payment and HMRC matches it to your record. If yours does not, the quicker route is to check that HMRC holds the right jobs and income for you, and to correct anything out of date, because that is what the new code will be built from.