Seeing M1 on your payslip can be confusing, and if you do not know what it means, it can quietly cost you money. The M1 tax code is one of the most commonly misunderstood emergency codes in the UK PAYE system. Understanding why you have it and how to fix it is straightforward once you know what to look for.
This guide covers everything you need to know about the M1 tax code, what it means, why HMRC applies it, and the exact steps to get it corrected.
What Is the M1 Tax Code in the UK?
The M1 tax code is an emergency tax code used within the UK’s PAYE system. It appears on your payslip alongside your standard tax code, for example, 1257L M1. The M1 suffix changes how your tax is calculated in a fundamental way.
Under a normal cumulative tax code, your employer calculates tax based on your total earnings and total tax-free allowance from the start of the tax year. This means any overpayments or underpayments earlier in the year are automatically corrected as the year progresses.
The M1 suffix removes this cumulative calculation entirely. Instead, tax is calculated on a month-by-month basis in isolation, as if each pay period is a fresh start. Previous months are ignored completely.
| Feature | Standard Cumulative Code | M1 Tax Code |
| Tax calculation basis | Cumulative, whole tax year | Month by month, each period is isolated |
| Previous months considered | Yes | No |
| Automatic self-correction | Yes | No |
| Overpayments corrected in-year | Yes | No |
| Triggered by | Normal circumstances | Missing information or new employment |
| Resolved by | N/A | Updating HMRC with the correct details |
The M in M1 stands for Month. It signals a monthly non-cumulative calculation. The equivalent weekly code is W1, Week 1, which works identically but on a weekly pay frequency basis.
How the M1 Tax Code Works
M1 switches your tax calculation from cumulative to non-cumulative. Each month is treated in isolation, no carry forward, no automatic corrections.
Cumulative Code: Your employer tracks total earnings and allowance used from April onwards. Unused allowance from earlier months carries forward automatically. Overpayments correct themselves through payroll.
M1 Code: Your employer applies exactly one twelfth of your annual allowance each month. Previous months are ignored entirely. Overpayments do not self-correct.
| Scenario | Cumulative Code | M1 Code |
| Monthly salary | £3,500 | £3,500 |
| Monthly tax-free allowance | £1,047 | £1,047 |
| Previous overpayment corrected | Yes, automatically | No, ignored |
| Year-end position | Accurate | Potentially overpaid |
The problem surfaces when pay has been irregular, you joined mid-year, or you had a gap in employment. A cumulative code corrects the imbalance automatically. M1 does not, and the overpayment builds silently until the code is fixed.
Most Common Reasons You Have Been Given an M1 Tax Code
There is always a specific reason for an M1 code. HMRC does not apply it carelessly, it appears when information is missing or when your employer cannot yet confirm your full tax position for the year.
Starting A New Job Without A P45
This is the most common trigger. When you begin a new role, your employer needs your P45 from your previous employer to set up the correct cumulative tax code. Without it, they apply an emergency code, typically 1257L M1, until HMRC issues updated instructions based on your actual circumstances.
Starting Your First Job
If you have never been employed before and have no P45, your employer has no prior year information to work with. An M1 code is applied until HMRC receives your details and issues the correct code.
Returning To Work After A Gap
A period of unemployment, self-employment, or time outside the workforce breaks the continuity of PAYE records. When you re-enter employment, your new employer may not have enough information to apply a cumulative code immediately.
Receiving Income From A New Pension
When a pension begins paying income for the first time, the pension provider may apply an emergency M1 code until HMRC confirms the correct tax treatment, particularly if you have other income sources that need to be factored in.
HMRC Issuing A Mid-Year Code Change
In some cases, HMRC issues a revised tax code mid-year with an M1 suffix to prevent a large underpayment or overpayment being corrected too abruptly through the payroll. This is an administrative decision rather than an emergency measure.
An Employer Payroll Error
If your employer’s payroll system loses or fails to process your tax code information correctly, an M1 code may be applied as a default. This is less common but does occur, particularly when businesses change payroll software or experience system migrations.
Whatever the cause, the fix is almost always straightforward. The sooner you identify the reason behind your M1 code, the sooner your tax position returns to where it should be.
How to Check or Correct Your M1 Tax Code
Correcting an M1 code is a straightforward process. The sooner you act, the sooner your tax deductions reflect your actual position for the year.
Step 1: Check Your Payslip
Look at your current tax code on your most recent payslip. If it shows M1, for example, 1257L M1, an emergency non-cumulative code is in place. Note the full code and the date it was first applied.
Step 2: Log In To Your HMRC Personal Tax Account
Visit gov.uk and access your Personal Tax Account using your Government Gateway login. Your current tax codes for all income sources are displayed here. Check whether the M1 code is shown and whether HMRC has the correct employment and income information on record.
Step 3: Provide Your P45 To Your Employer
If the M1 code was triggered by starting a new job without a P45, obtain it from your previous employer immediately and hand it to your new employer’s payroll team. This is the fastest resolution route. Your employer will update your tax code based on the P45 information and notify HMRC.
Step 4: Complete A New Starter Checklist If No P45 Is Available
If you do not have a P45, for example, because you were previously self-employed, had a career break, or are starting your first job, complete your employer’s new starter checklist accurately. Select the correct statement that reflects your employment history. This gives HMRC the information needed to issue the correct code.
Step 5: Contact HMRC directly
If the M1 code has been in place for more than one pay period and your employer has not yet received an updated code, contact HMRC directly on 0300 200 3300 or through the secure messaging function in your Personal Tax Account. Provide your National Insurance number, employer details, and the date your employment started. HMRC will issue a revised code to your employer.
Step 6: Claim Any Overpaid Tax
If the M1 code has resulted in overpaid tax, HMRC will typically refund this automatically at year-end through the P800 reconciliation process. If you want the refund sooner, contact HMRC to request an in-year adjustment. Do not wait until April if the overpayment is significant.
Most M1 code issues resolve within one or two pay periods. Once the right information reaches HMRC, the steps above are all it takes.
Common Mistakes Employees Make When Dealing With an M1 Tax Code
These errors consistently prolong the problem and cost employees money they are entitled to recover.
- Ignoring the M1 suffix on their payslip: Many employees notice their tax code but overlook the M1 suffix entirely. The suffix changes how tax is calculated, not just the allowance applied. Ignoring it means missing an opportunity to correct your tax position early in the year.
- Assuming HMRC will fix it automatically: HMRC does not always detect and correct an M1 code without input from you. If the information gap that triggered the code has not been resolved, the M1 designation can persist for months. Proactive contact with HMRC is almost always faster than waiting.
- Not providing a P45 to the new employer promptly: The P45 is the fastest and most direct route to resolving an M1 code. Many employees delay handing it over or forget to request it from their previous employer. Request your P45 on your last day and provide it to your new employer on day one.
- Completing the new starter checklist incorrectly: The checklist contains three statement options that reflect your employment history. Selecting the wrong statement, for example, claiming this is your only job when you have another, produces an incorrect tax code. Read each option carefully before selecting.
- Not following up after submitting information: Providing your P45 or completing a starter checklist does not always result in an immediate code update. Follow up with your payroll team to confirm the updated code has been received and applied. Do not assume the process has completed without verification.
- Waiting until year-end to recover overpaid tax: You do not have to wait for the P800 year-end reconciliation to recover tax overpaid under an M1 code. Contact HMRC and request an in-year repayment. For significant overpayments, acting promptly makes a material difference to your cash flow.
Every one of these mistakes is avoidable. A few proactive steps taken early save you months of overpayment and the effort of recovering tax that should never have left your pay packet.
Final Thoughts
The M1 tax code is temporary, it exists because HMRC lacks the information needed to apply your correct cumulative code. Once that information is in place, M1 is removed, and your tax calculation returns to normal.
Act quickly. Provide your P45, complete your starter checklist accurately, and follow up with HMRC if the code has not updated within a pay period. If M1 has been in place for several months, contact HMRC directly. You are entitled to recover every pound overpaid.
FAQs
What Does M1 Mean On a Tax Code?
M1 stands for Month 1, it means your tax is calculated on a monthly basis in isolation, without reference to the rest of the tax year. It is an emergency designation applied when HMRC does not have enough information to issue your correct cumulative code.
Does An M1 Tax Code Mean I Am Paying Too Much Tax?
Not necessarily, in straightforward cases with consistent monthly income, M1 and a cumulative code produce similar deductions. The risk of overpaying increases when your income varies month to month or when you have a complex mid-year tax position that would normally be corrected cumulatively.
How Long Does It Take To Fix An M1 Tax Code?
Once HMRC receives the correct information, through a P45, a completed starter checklist, or direct contact, a revised code is typically issued within a few weeks. Your employer applies the updated code from the next available payroll run.
Will I Get a Refund If I Overpaid Tax Under An M1 Code?
Yes, HMRC reconciles PAYE payments at the year-end through the P800 process. If you have overpaid, a refund will be issued automatically. You can also contact HMRC to request an in-year repayment rather than waiting until April.
What Is The Difference Between M1 And W1?
Both are non-cumulative emergency codes, M1 applies to monthly paid employees, and W1 applies to weekly paid employees. They work identically in terms of how tax is calculated, in isolation, each pay period, without reference to the rest of the year.
Can I Have An M1 Code On A Second Job?
Yes, if you have a second job and your employer does not have the correct tax information, an M1 code may be applied to that income source. The correct code for a second job is typically BR, basic rate with no personal allowance. If M1 is applied incorrectly to a second job, contact HMRC to correct it.
