It is the code that forgets. Each payday it taxes you as if the year just began, which is exactly why so many people on it quietly overpay and have to claim the money back.
Somewhere on a busy new starter's payslip sits a tiny suffix, M1 or W1, that quietly changes everything. It tells payroll to forget the rest of the year and tax this payday as if it stands alone. The result is one of the most common ways UK workers overpay tax without ever realising it.
A non-cumulative code is the opposite of the default cumulative one. Rather than tracking your running total since 6 April, it resets every payday. On a monthly M1 code, you get one twelfth of your Personal Allowance, £1,047.50 in 2025/26, applied to that month and that month only. On a weekly W1 code, you get one fifty-second, about £241.73.
Crucially, the code does not know or care what you earned in previous periods. If you earned nothing last month, that unused allowance does not roll forward. If you overpaid two months ago, it will not be returned. Each payslip starts from scratch.
The trouble with a code that forgets is that real working lives are uneven. People change jobs mid-year, take unpaid leave, or have a quiet month followed by a busy one. A cumulative code absorbs all of that by averaging across the year. A non-cumulative code cannot.
If you start a new job in July, having used only part of your allowance in your old job, a Week 1/Month 1 code gives you a fresh single-period slice each month and never reconciles it against your earlier earnings. You may be giving away allowance you have already used, or, just as often, paying tax you should not owe. Until HMRC has the full picture, the imbalance keeps building.
Marcus left a job in June 2025 having earned £15,000 and used £3,141 of his allowance (three months). He starts a new role in July on £3,000 a month, but cannot find his P45, so HMRC issues 1257L M1, a non-cumulative emergency code.
Each month from July, payroll gives him exactly £1,047.50 of allowance and taxes the remaining £1,952.50 at 20% (£390.50), never accounting for the allowance he already used or the months he was not paid. Because the code ignores his actual year-to-date position, his tax may be too high or too low depending on his full-year earnings, and it will not balance out on its own.
Once Marcus submits his details and HMRC issues a cumulative 1257L, his next payslip recalculates the whole year and corrects the difference. The check my tax code walkthrough helps confirm whether you are owed money while stuck on a W1/M1 code.
A cumulative code learns from your whole year. A non-cumulative code wakes up every payday with amnesia.
Being on a non-cumulative code is not a penalty and is often temporary, but it is worth acting on. Give your employer your P45 from your previous job, or log in to your Personal Tax Account on gov.uk to confirm HMRC has your correct employment details. Once a cumulative code is issued, the system reconciles your year and refunds any overpayment through payroll, usually within one or two pay runs. If the tax year ends before you are switched back, HMRC reconciles it automatically and issues a P800 with any refund due.
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