If you have spotted “M1” on your payslip or HMRC letter and have no idea what it means, you are not alone. The M1 tax code is a temporary code that HMRC uses when they do not have enough information about your income to apply the correct tax. In simple terms, it means you are being taxed on a month-by-month basis rather than across the full tax year, which could leave you paying too much or too little tax.
What Does the M1 Tax Code Mean?
The M1 tax code stands for “Month 1”. It is what HMRC calls a non-cumulative tax code, meaning your employer calculates your Income Tax based only on what you earn in that specific month, without looking at what you have already earned or paid in tax since April (the start of the UK tax year).
Under a normal cumulative tax code (like the standard 1257L), your employer tracks your earnings and tax payments from the start of the tax year. This ensures you use your full Personal Allowance evenly and pay the right amount of tax overall.
With M1, that running total is reset each month. Your employer only knows about that month’s pay — nothing before it. This can lead to overpayments or underpayments depending on your situation.
Quick Example: If you earn £3,000 in a month, your employer applies 1/12th of your annual Personal Allowance (£1,048) and taxes the rest. They do not consider what happened in previous months. This continues until HMRC updates your code.
Why Do You Get the M1 Tax Code?
HMRC does not assign the M1 tax code randomly. There are a number of common reasons you might receive it.
Have You Recently Started a New Job?
Starting a new job is one of the most common triggers for M1. If your new employer does not receive a P45 from your previous employer in time, they cannot work out how much tax you have already paid. To avoid getting it wrong, HMRC or your employer may apply M1 as a holding code until the correct information comes through.
Have You Just Come Off an Emergency Tax Code?
Emergency tax codes often end with M1 or W1. When HMRC processes your information and moves you off an emergency code, they may still use M1 temporarily while your records are being updated.
Has Your Tax Code Recently Changed?
Sometimes HMRC changes your tax code part-way through the tax year — for example, if you start receiving company benefits, change jobs, or your income changes. When a code changes mid-year, HMRC may use M1 to stop your employer recalculating all the tax from April 1st again, which could cause a sudden underpayment or overpayment.
Are You Self-Employed or Have Multiple Income Sources?
If you are self-employed but also employed, or if you have more than one PAYE income, HMRC sometimes applies M1 to one of your employments while they sort out how to split your Personal Allowance correctly.
Were You Previously on a Career Break or Took Time Off?
If you have been out of employment for a while and are returning to work, HMRC may not have up-to-date earnings information on file. M1 is used to tax you safely until they can confirm your situation.
What is the Difference Between M1 and W1?
You might also come across W1, which stands for “Week 1.” The difference is simple:
| Code | Stands For | How Tax is Calculated |
|---|---|---|
| M1 | Month 1 | Based on monthly earnings only |
| W1 | Week 1 | Based on weekly earnings only |
Both are non-cumulative codes that work the same way — they just apply to different pay frequencies. If you are paid monthly, you will usually see M1. If you are paid weekly, you will usually see W1.
How Does M1 Tax Code Affect Your Take-Home Pay?
This depends entirely on your situation. The M1 tax code is not always bad — but it can result in the wrong amount of tax being deducted.
Can M1 Tax Code Mean You Pay Too Much Tax?
Yes. If you earned less earlier in the year (or had a gap in employment), a cumulative code would take that into account and reduce your tax. M1 does not do that. It assumes you earn the same amount every month, which means you could be paying more tax than you owe.
Can M1 Tax Code Mean You Pay Too Little Tax?
Also yes. If your income is irregular or higher than HMRC expects, M1 may not deduct enough tax. In this case, HMRC will catch the underpayment at the end of the year and collect it — usually through a future tax code adjustment or a Simple Assessment bill.
Will You Get a Tax Refund?
If you have overpaid tax because of M1, you are entitled to a refund. HMRC usually sorts this out automatically at the end of the tax year, but you can also contact HMRC directly to claim it sooner.
How to Check Your Tax Code
Before you do anything else, it helps to know what tax code you are actually on. Here is where to look:
- Your payslip — Your tax code will be printed on every payslip, usually alongside your National Insurance number.
- Your P60 — This annual summary from your employer will show the tax code used at the end of the year.
- Your HMRC Personal Tax Account — You can log in at gov.uk/personal-tax-account and check your current tax code online.
- A letter from HMRC — If your code has changed, HMRC usually sends a PAYE Coding Notice (P2) to let you know.
How Do You Fix the M1 Tax Code?
In most cases, HMRC will update your tax code automatically once they receive all the information they need. However, the process can take time — and in the meantime, you could be paying the wrong amount.
Should You Contact HMRC?
Yes, if you have been on M1 for more than a few weeks and nothing has changed, it is worth getting in touch. You can contact HMRC’s Income Tax helpline on 0300 200 3300 (Monday to Friday, 8 am to 6 pm). Have the following ready:
- Your National Insurance number
- Your employer’s PAYE reference (found on your payslip)
- Details of your income or any recent changes to your employment
What Will HMRC Do?
Once HMRC has the information they need, they will issue a new tax code to your employer. Your employer then applies the updated code from the next payroll run. If you overpaid under M1, the new code will usually give you credit for this — reducing the tax on future pay to balance things out.
Is M1 the Same as an Emergency Tax Code?
Not exactly. Emergency tax codes and M1 are related, but they are not the same thing.
An emergency tax code is what HMRC uses when they have very little information about you — for example, if you have never been in PAYE employment before, or if you have not provided your P45. Emergency codes often look like 1257L M1, 1257L W1, or BR (Basic Rate).
M1 is the suffix added to a tax code (like 1257L M1) to make it non-cumulative. So a code can be both an emergency code and carry the M1 suffix — they serve slightly different purposes but often appear together.
What Does 1257L M1 Mean?
1257L M1 is one of the most commonly seen versions of the M1 code. Here is what each part means:
- 1257 — This reflects the standard Personal Allowance for 2026/27, which is £12,570. HMRC removes the last digit, so 12,570 becomes 1257.
- L — This means you are entitled to the standard tax-free Personal Allowance.
- M1 — This confirms the code is non-cumulative and calculated on a month-by-month basis.
So 1257L M1 means your employer is giving you 1/12th of your £12,570 allowance each month and taxing the rest — but without considering your full year’s earnings.
FAQs About the M1 Tax Code
How Long Does M1 Tax Code Last?
There is no fixed length. It stays in place until HMRC updates your records and issues a new code. This could take a few weeks or, in some cases, a few months. If you think it has gone on too long, contact HMRC.
Does M1 Affect My National Insurance Contributions?
No. National Insurance is calculated separately from Income Tax and is not affected by your tax code, including M1.
Can My Employer Change My Tax Code?
No. Your employer can only use the tax code provided by HMRC. They cannot change it themselves. Any update to your code must come from HMRC directly.
What If I Have Two Jobs and One Shows M1?
If you have more than one employer, HMRC may apply your full Personal Allowance to your main job and use a different code — sometimes BR or 0T — on your second job. The M1 suffix may appear on either of these. If you are unsure how your allowance is being split, speak to HMRC or an accountant.
Will M1 Show on My P60?
Your P60 will show the tax code that was in place at the end of the tax year. If M1 was still active, it would appear on your P60. However, once the year is over, HMRC will reconcile your tax and issue any refund or adjustment that is owed.
What Should You Do If You Are Worried About Your Tax Code?
Tax codes are easy to overlook — most people just glance at their take-home pay and move on. But an incorrect tax code can silently cost you money throughout the year.
Here is a simple checklist to follow:
- Check your payslip — Look for M1 next to your tax code.
- Log in to your HMRC Personal Tax Account — Confirm what code is currently active.
- Make sure HMRC has the right information — Particularly if you have changed jobs, started a new role, or had a break in employment.
- Contact HMRC if the code has not changed — Do not wait for the year-end if you suspect you are overpaying.
- Speak to an accountant — If your situation is more complex (multiple incomes, self-employment, or ongoing tax code issues), getting professional advice can save you significant time and money.
Need Help with Your Tax Code?
At Right Choice Consulting, we work with business owners and self-employed individuals across the UK who are dealing with exactly these kinds of tax issues. Whether it is an incorrect PAYE code, an overpayment from HMRC, or simply trying to understand what your payslip actually means — our team is here to help.
We take care of the complexity so you can focus on running your business.
Get in touch with our team today — or explore our other guides to understand the UK tax system better.
Summary: Key Things to Remember About the M1 Tax Code
- M1 stands for “Month 1” and means your tax is calculated on a monthly, non-cumulative basis.
- It is commonly applied when you start a new job, come off an emergency tax code, or HMRC is missing information.
- It can cause you to overpay or underpay Income Tax — neither outcome is ideal.
- In most cases, HMRC updates the code automatically, but you can contact them to speed things up.
- If you are unsure whether you are on the right code, checking your HMRC Personal Tax Account is the quickest first step.
Disclaimer: All the information provided in this article is general in nature and it does not intend to disregard any of the professional advice.