Wrong Tax Code? How to Check and Fix It [Full Guide]

A wrong tax code means HMRC is using incorrect information about your income, benefits, or allowances.

Your tax code controls how much Income Tax gets taken from your pay each month. If HMRC has the wrong information, you could be paying too much tax without realising it, or building up a debt that will need to be repaid later.

This guide explains how tax codes work, the signs that yours might be wrong, and what to do to fix it.

Key points

• Emergency tax codes can cause you to overpay ⚠️
  Codes like W1, M1, and OT are temporary emergency codes used when HMRC lacks information, often after starting a new job without a P45. OT is usually the most expensive because it removes your tax-free allowance.
• Wrong tax codes are more common after life or job changes 🔄
  Starting a new job, having multiple sources of income, receiving workplace benefits, or changing Marriage Allowance details can all trigger the wrong code if HMRC's records are outdated or incomplete.
• Small warning signs can help you catch mistakes early 👀
  A sudden change in take-home pay, unfamiliar letters in your code, or unexpectedly high tax deductions are all signs that your tax code may need checking. Reviewing payslips and your Personal Tax Account regularly can prevent bigger problems later.
• ANNA helps you spot tax problems before they become expensive 📱
  If you manage freelance income, self employment, or a business alongside PAYE work, ANNA helps you track income, estimate taxes in real time, automate bookkeeping, and stay on top of HMRC changes so tax code mistakes are easier to catch and fix.

What is a tax code, and how does it work?

A tax code is a short combination of numbers and letters that HMRC sends to your employer or pension provider. It tells them how much of your income to leave untaxed before applying the relevant rate.

For most people in England, Wales, and Northern Ireland, the standard code is 1257L. The number represents your tax-free Personal Allowance of £12,570 (divided by 10), and the letter tells your employer how to apply it.

However, that's not the only combination possible.

Here's what the most common letters mean:

The number in your code can also shift if you have taxable benefits through work, like a company car or private medical insurance. The value of those benefits reduces your effective tax-free amount.

Emergency tax codes

HMRC applies emergency codes when it doesn't have enough information to determine your correct code, typically when you start a new job without providing a P45 form.

The most common emergency codes are:

• 1257L W1 (taxed on a week-by-week basis)
• 1257L M1 (taxed on a month-by-month basis)
• OT (no Personal Allowance given at all)

W1 and M1 tax codes aren't always incorrect, but they are temporary. They calculate tax only on that week's or month's pay and ignore what you've already earned or paid earlier in the tax year.

This can lead to overpaying tax, especially if you started a job partway through the year and hadn't used much of your Personal Allowance by then.

OT codes are usually more expensive because they give you no Personal Allowance, which means that all your earnings from that job are taxed, not just the amount above £12,570.

To fix an emergency tax code, give your employer your P45 from your previous job as soon as possible. If you don't have one, complete the starter checklist or contact HMRC so the correct code can be issued.

How do you get a wrong tax code?

HMRC relies on information from you, your employer, and other government departments to set your code. When that information is incomplete or out of date, the code can be wrong.

Common causes include:

• Starting or leaving a job: If your new employer doesn't have your P45 or submits incorrect starter declaration details, HMRC may apply an emergency code while it works out what you should be paying.
• Multiple jobs or pensions: Each income source gets its own code. If HMRC isn't told about all of them, your allowances might be applied against the wrong income or duplicated.
• Taxable benefits changing: If you stopped using a company car but didn't notify HMRC, your code might still be deducting it from your allowance.
• Marriage Allowance transfers: If you've transferred or received the Marriage Allowance but the claim hasn't been processed yet, your code won't reflect it.
• Self Assessment adjustments: HMRC sometimes collects smaller underpayments through your tax code rather than a direct bill. If this adjustment is wrong, it will affect your code.
• Claims that have expired or lapsed: Expense claims, professional subscriptions, or working-from-home relief may have been included in a previous year's code but require annual renewal.

Signs your tax code might be wrong

You won't always get a letter when your code changes. Checking your payslips and personal tax account regularly is the only reliable way to catch an error early.

Here are a few things to look out for:

• Your pay changes unexpectedly, but your salary doesn't: If your take-home amount drops or rises without an obvious reason, your tax code may have been updated.
• You've recently changed jobs, and you're paying a lot of tax: Emergency codes like W1/M1 or OT mean you're not getting your full allowance, and you'll typically pay more than you should until the code is fixed.
• You receive a PAYE Coding Notice (P2) that looks unfamiliar: HMRC sends these when your code changes. If the adjustments listed don't make sense, that's a signal that something needs checking.
• You have more than one job, and one source seems undertaxed: If all your allowance is being applied to a low-earning role while a higher-earning one is taxed at the basic rate, you could end up with a surprising bill at year's end.
• Your code has the letter K, and you don't know why: A K code can be given for legitimate reasons, but if you don't know why it would apply to you, you should check with HMRC.

How to check your tax code

Here are several ways to find and verify your current code:

• Your payslip: Your tax code appears on every payslip alongside your gross pay and deductions.
• Your P60 or P45: Your P60, issued at the end of each tax year, shows the code used for that year. If you've left a job, your P45 will show the code as of the date you left.
• HMRC's Personal Tax Account: You can log into your Personal Tax Account to see your current code, a breakdown of how it's been calculated, and any adjustments HMRC has made.
• The HMRC app: It lets you check your code on your phone and provides the same information as your online personal tax account.
• By phone: If you'd rather speak to someone, you can call HMRC's income tax helpline on 0300 200 3300. Have your National Insurance number ready for identification.

Once you've found your code, multiply the number by 10 to get the tax-free amount it represents.

💡 Did you know?

If you run your own business or have self employed income alongside employment, ANNA's Auto Accountant can pull your records together and estimate what you owe across all your income streams, so there are no surprises when tax season arrives.

How to fix a wrong tax code

Once you've identified a problem, HMRC can usually correct it quickly.

Here's what to do depending on how you want to get in touch:

Online via your Personal Tax Account

• Log in to your Personal Tax Account
• Go to Check your Income Tax
• Select Tell HMRC about a change and update the relevant information, such as removing a benefit that no longer applies or correcting employment details

HMRC will then recalculate your code and notify your employer. This is usually the fastest route for straightforward corrections.

By phone

Call HMRC's income tax helpline on 0300 200 3300. Lines are open Monday to Friday, 8am to 6pm.

Be ready to explain why you think the code is wrong, and prepare any supporting details, such as payslips, a P11D form showing the value of any benefits, or a letter from your employer.

Through your employer

If the issue relates to how your employer has reported your pay or benefits to HMRC, it's worth raising it with your payroll team first. They can correct submissions and resubmit information, which then triggers an update from HMRC.

What happens next

Once HMRC updates its records, it'll send you a new P2 coding notice and pass the updated code to your employer. Your employer will usually apply it from the next payroll run.

If you've been paying too much, the adjustment will typically give you a higher tax-free amount temporarily to offset what you've overpaid throughout the year.

🧠 Good to know:

HMRC aims to process straightforward code changes quickly, but if the change involves Self Assessment or more complex income, it can take longer. Keep an eye on your payslips to confirm when the correction takes effect.

How ANNA can help you stay on top of your tax code

ANNA is an all-in-one business app that can register your business, send and track invoices, do your bookkeeping, and file your VAT, Corporation Tax, and MTD Self Assessment.

If you're self employed, a sole trader, or a company director drawing a salary alongside business income, keeping a real-time picture of your earnings makes it much easier to see whether your PAYE code reflects your actual situation.

Here's what ANNA can offer you:

• Real-time tax estimates: Lets you see how much you owe to HMRC at any time, so you can catch differences between your expected tax and your tax code implications before they become a problem.
• Automatic expense capture and categorisation: Ensures every transaction is recorded correctly, which matters when HMRC adjusts your code to collect estimated underpayments based on your Self Assessment return.
• Automated MTD Income Tax Self Assessment: Calculates your taxes, prepares your documents, and files directly under Making Tax Digital rules, so any overpayments or underpayments show up at the right time rather than building up unnoticed.
• Smart money pots: Automatically sets aside a percentage of your income for tax, so you're not caught short if your code changes and an unexpected bill follows.
• Built-in UK business account: Lets you manage cards, transfers, and payments all in one place, giving you a single source of truth for your income records.

Sign up for ANNA today and take the stress out of tax, invoicing, and bookkeeping.