An M1 tax code is a UK emergency tax code assigned by HMRC. It is used when you are paid monthly and HMRC lacks complete details about your income. It is a temporary “non-cumulative” code, meaning taxes are calculated based only on that month’s earnings. It does not consider your overall year-to-date income or tax-free allowance.
Being on the M1 tax code generally means you could be paying more tax than necessary. That is why it’s so important to understand what it means and how to fix it.
In this guide, you’ll get to know:
- What is the M1 tax code UK?
- Why have I been given a tax code M1,
- How can I change my M1 tax code,
- And much more…
Let’s get into it!
What Is the M1 Tax Code?
The M1 tax code is an emergency tax code used by HMRC when they don’t have enough information to apply your regular tax code. This can happen if you start a new job, have multiple jobs, or haven’t updated your tax details with HMRC. The M1 stands for Month 1. It is mainly used for people who are paid monthly or for one-off payments, like a pension withdrawal.
What Does Tax Code M1 Mean?
Tax code M1 means your tax is being calculated using only 1/12th of your personal tax-free allowance for that specific month, completely ignoring any earnings or unused allowances from earlier in the tax year.
In simpler terms, being on the M1 tax code means you could be paying more tax than necessary. Because your tax-free allowance is not being considered properly.
Cumulative vs Non-Cumulative Tax Codes
The difference between Cumulative vs Non-Cumulative Tax Codes is below:
- Cumulative tax codes: The standard UK system. It calculates tax based on your total year-to-date earnings and applies your accumulated personal allowance across the elapsed months.
- Non-cumulative tax codes: Temporary measures like M1. They operate on a “week 1” or “month 1” basis, treating each payday as if it is the very first week or month of the financial year.
How Does the M1 Tax Code Affect Your Take-Home Pay?
An M1 tax code can cause you to overpay tax if your income varies or if you had a gap in employment earlier in the tax year. Because it calculates tax for each month in complete isolation, it cannot factor in the tax-free personal allowance you missed out on during those jobless months. This results in a lower take-home pay than you are actually owed.
Example of 1257L M1 vs Standard 1257L
Imagine you did not work in April, but you started a job in May earning £3,000 gross per month.
- Standard 1257L (Cumulative): Your payroll looks at May and notices you have two months’ worth of Personal Allowance available (£2,096 total tax-free) because you did not use your April allowance. You are only taxed 20% on the remaining £904.
- 1257L M1 (Non-Cumulative): Your payroll is forced to ignore April. It gives you just one month of Personal Allowance (£1,048) and taxes you at 20% on the entire remaining £1,952. Your take-home pay is lower this month because your unused April allowance is temporarily frozen.
What is the Difference Between M1, W1, and X Tax Codes?
All three symbols mean the exact same thing to HMRC: you are on an emergency, non-cumulative tax code. The only difference is the payment schedule your employer uses to calculate your pay.
Here is exactly how they differ across pay intervals:
- M1 (Month 1): This code is applied if you are paid monthly. Your payroll software divides your annual tax-free allowance by 12 and calculates your tax based solely on that single month’s earnings.
- W1 (Week 1): This code is applied if you are paid weekly. Your payroll software divides your annual tax-free allowance by 52 and looks at that specific week’s earnings in isolation.
- X (Generic Indicator): This is a universal, non-cumulative marker. HMRC or your payroll software uses “X” if you have an irregular pay cycle (such as four-weekly, fortnightly, or casual shift work) where M1 or W1 do not neatly apply.
Why Have I Been Given the M1 Tax Code?
The biggest question people ask is often, “Why have I been given a M1 tax code?”
Well, it doesn’t mean you have done anything wrong. It just means HMRC is missing some information about your tax history for the current year.
The most common reasons for getting the M1 tax code include:
1. Starting a New Job Without a P45
When you leave a job, your previous employer issues a P45 showing your year-to-date earnings and tax paid. If you start a new job without handing this document to your new employer, their payroll system cannot guess your previous financial history.
To prevent you from underpaying tax, they must put you on an emergency M1 tax code. This is done until HMRC automatically sends them your official tax code.
2. Changing Jobs Frequently or Having Multiple Incomes
If you change employers multiple times in a short window, the payroll data sent to HMRC often overlaps. HMRC’s automated systems may temporarily assume you are holding down multiple jobs at once.
Alternatively, if you actually do have two or more active sources of income, HMRC will often place an M1 tax code on the secondary income. This is done in order to ensure your full personal allowance is not accidentally applied to both paychecks. Because this would cause a massive tax debt later.
3. Returning to Work After a Career Break
If you return to employment after a period of self-employment, state benefits, or a completely unpaid career break, HMRC’s system registers a gap in your PAYE (Pay As You Earn) history.
Because the system cannot immediately verify whether you earned taxable income during that gap, it defaults to a non-cumulative M1 code to reset your monthly tracking safely.
4. Receiving Company Benefits for the First Time
When you get a new company perk, your tax-free personal allowance must be reduced to pay for that benefit. If HMRC learns about the benefit mid-year, they will apply an M1 code to your salary.
This prevents your employer from taking a massive retrospective chunk of tax from a single paycheck to cover prior months. Yes, instead of spreading the new tax cost evenly across your remaining paychecks.
In all these cases, the M1 tax code is usually a temporary measure. Once HMRC has all the necessary information, they’ll switch you back to your regular tax code.
Example Of What The M1 Tax Code Looks Like On Your Payslip
On your payslip, the tax code is usually found near your name or employee number. Under a heading like Tax Code or T Code.
It will look something like this:
- 1257L M1 (For the 2026/27 tax years, 1257L is the standard personal allowance code for England and Northern Ireland. The M1 is the key part that changes how it works).
- S1257L M1 (The ‘S’ prefix indicates you pay Scottish Income Tax rates).
- C1257L M1 (The ‘C’ prefix indicates you pay Welsh Income Tax rates).
- SK125 M1 (The ‘K’ prefix means your taxable benefits or owed tax exceed your allowance, and the ‘S’ indicates you pay Scottish Income Tax rates).
- 0T M1 (Tax code 0T M1 means zero personal allowance is being used for that month).
The crucial thing to look for is the M1 or Month 1 identifier. This confirms your tax is being calculated on a non-cumulative, emergency basis.
How to Check If You’re Actually on M1
To check if you are on M1 tax code, have a look at:
- Your latest payslip, usually printed near your name or employee number
- Your HMRC Personal Tax Account at gov.uk, which shows your current live code
- Any P2 Coding Notice HMRC has sent you by post or online
If you see M1 next to your tax code in any of these places, that confirms it.
How Can I Change My M1 Tax Code?
The good news is that the M1 tax code is temporary and relatively easy to fix. Your goal is to get HMRC to issue a corrected, cumulative tax code.
Provide Your P45 to Your Employer
If you’ve recently changed jobs, the quickest fix is usually to provide your new employer with your P45 form from your previous job immediately.
This document has all the crucial tax information they need to apply the correct code. If you don’t have a P45, you should fill in the Starter Checklist.
Contact HMRC Directly
You can update HMRC in either of these ways:
- Use Your Personal Tax Account: The best method is to log in to your Personal Tax Account on the GOV.UK website or via the HMRC app. Here you can check your current tax code. You can update employment details and inform HMRC of any changes to income or benefits.
- Call HMRC: You can call the HMRC Income Tax helpline on 0300 200 3300. Make sure you have your National Insurance number, the M1 tax code shown on your payslip, and any P45s/P60s to hand. The phone lines are open Monday to Friday, 8am to 6pm.
Most people who overpay under an M1 code get it back automatically once the correct cumulative code is applied later in the same tax year. If the tax year ends before that happens, HMRC will usually issue a refund after doing its annual reconciliation, though you can also claim directly if you would rather not wait.
Once you provide the correct details, HMRC will review your records and issue a new, cumulative tax code to both you and your employer. This is called a PAYE Coding Notice (or P2).
Check Out: How to Change Tax Code Online?
What Happens After the Code is Corrected?
When your employer receives the new code, they will apply it in the next available payroll run for the upcoming pay period. Because the new code is cumulative, it will automatically look back at the pay and tax you’ve had since 6 April and calculate whether you’ve paid too much tax while on the M1 tax code.
If you overpaid tax, the excess amount will usually be refunded in a future payslip. This will result in a much larger take-home amount that month!
If you remain on the M1 code until the end of the tax year (5 April), HMRC will usually reconcile your tax position automatically and send you a P800 form. If the P800 shows you are owed a refund, you will then need to claim it. You can do it via your Personal Tax Account or the HMRC app.
Other Emergency Tax Codes You Might Encounter – 2026/2027
It’s also useful to know about other tax codes you might come across, especially if you’ve had a change in employment. Here are a couple of other codes you might see:
For the 2026/2027 tax year, the default emergency code is 1257L, with a specific suffix indicating the emergency basis.
- 1257L W1: Used if you are paid weekly. W1 stands for ‘week 1’.
- 1257L M1: Used if you are paid monthly. M1 stands for ‘month 1’.
- 1257L X: Used for non-standard pay periods.
- BR: Basic Rate. This code is used when you have a second job or a pension. And all your income is taxed at the basic rate (20% for 2026/27) with no tax-free allowance.
- 0T: 0T tax code means your entire income is taxed, with no personal allowance. This tax code is often applied when HMRC doesn’t have enough information about your tax status, similar to the M1 code.
Will I Get a Tax Rebate Automatically After Fixing My M1 Code?
Yes, you will usually receive your tax rebate automatically through your next payroll once your M1 tax code is fixed. However, if fixed before April 5th, your employer refunds you on your next payday. If fixed after April 5th, HMRC handles it after calculating your end-of-year tax. They will send a P800 letter between June and November. You must then log into your Government Gateway account to claim the cash online.
How Long Does It Take HMRC to Update an Emergency Tax Code?
It typically takes HMRC up to 35 days to automatically update an emergency tax code after your new employer registers you on their payroll.
However, the exact timeline depends entirely on how the issue is handled:
- The Automatic Route: If you are waiting for systems to talk to each other, it takes up to 35 days. This is from your first payday for HMRC to receive your P45 or starter details and issue a correction.
- The Manual Online Route: If you log into your HMRC Personal Tax Account and manually report a wrong tax code, HMRC aims to update it and notify your employer within 15 working days.
- The Phone Route: If you call HMRC directly at 0300 200 3300, an adviser can often update your code instantly over the phone. Yes, if they have all your income details.
Once HMRC updates the code, it takes about 7 days for the digital notice to reach your company’s payroll. You will then see the adjusted code and your refund on your very next payday.
Does the M1 Tax Code Affect My National Insurance Contributions?
No, the M1 tax code has absolutely no impact on your National Insurance contributions. Tax codes are exclusively used by employers to calculate Income Tax deductions. National Insurance is worked out independently using completely separate, fixed statutory thresholds that apply equally to every employee regardless of their current tax code status.
What Do the Letters Mean in Tax Codes?
The letters in the M1 tax code indicate the amount of personal and tax-free earnings allowance that an individual has remaining for the current tax year. The letters used in the tax code are:
- T – Indicates that the individual has exhausted their tax-free personal allowance for the tax year.
- A – Indicates that the individual has exhausted their tax-free earnings allowance for the tax year.
- 0 – Indicates that the individual has neither exhausted their tax-free personal allowance nor their tax-free earnings allowance for the tax year.
- 1 – Indicates that the individual has exhausted their tax-free earnings allowance but not their tax-free personal allowance for the tax year.
- B – Indicates that the individual has exhausted their tax-free earnings allowance, but they have tax-free earnings allowance remaining from a previous tax year.
- K – Indicates that the individual has unused tax-free earnings allowance, but they have an income tax or employee tax due this tax year.
- M – Indicates that the individual has tax-free earnings allowance and unused tax-free allowance, but they have income tax or employee tax due this tax year.
The Bottom Line
The M1 tax code is an emergency tax code used by HMRC when they don’t have enough information about your tax situation. While it’s temporary, it can result in overpaying tax.
Therefore, if you’ve been assigned an M1 tax code, it’s important to contact HMRC. So you can switch back to a correct tax code faster, avoiding an unnecessary dent in your monthly budget.
HOW ACCOTAX CAN HELP?
At Accotax, we deal with tax code issues for clients regularly, so we know how to get things sorted quickly with HMRC.
We’ll check your code, work out if you’re due a refund, and contact HMRC on your behalf so you don’t have to. If you’ve also got a second job or self-employed income, we can review that alongside it.
Reach out, get an instant quote and let us help you stay compliant!
Disclaimer: All the information provided in this article on “what is the M1 tax code“, including all the texts and graphics, is general in nature. It does not intend to disregard any of the professional advice.
