What Is a Tax Code and How to Check Yours (UK)

Photo by Chris Karidis on Unsplash

Your tax code is the number that tells your employer — or pension provider — how much income tax to deduct from your pay. Most people never think about it, which is exactly why many people end up paying the wrong amount of tax for months or even years without realising.

Getting your tax code wrong can mean overpaying tax (which HMRC will eventually refund, but not always proactively) or underpaying it (which results in a surprise tax bill later). This guide explains what tax codes are, what yours means, and exactly how to check whether it’s correct.

What Does a Tax Code Actually Do?

Your employer uses your tax code to work out how much tax-free income you can receive before tax kicks in. The most common tax code in 2026 is 1257L, which tells your employer that you have a tax-free personal allowance of £12,570.

The way it works:

• Take the number in your code (e.g., 1257) and add a zero → £12,570
• This is the amount you earn before paying any income tax
• Everything above that is taxed at the appropriate rate (20% basic, 40% higher, 45% additional)

The letter at the end modifies how that allowance applies.

What the Letters in Your Tax Code Mean

The Most Common Tax Codes

1257L is the standard code for most UK employees and pensioners in 2026. It reflects the current Personal Allowance of £12,570.

BR means you’re being taxed at the flat 20% basic rate on everything from that particular income source — no allowance applied. This is normal for second jobs or a second pension, but wrong if it’s your main income.

1257L W1 or 1257L M1 — the “Week 1” or “Month 1” suffix means HMRC is using a non-cumulative calculation, treating each pay period independently rather than assessing your total earnings to date. This sometimes happens when you start a new job mid-year. It’s not necessarily wrong, but it can result in either over- or under-payment during the year.

K codes are less common and typically arise when you have benefits in kind (like a company car), state pension, or other untaxed income that exceeds your allowance. The K means your employer deducts extra tax rather than applying an allowance.

How to Check Your Tax Code

You can check your current tax code in several ways:

1. Your payslip — your tax code should appear on every payslip. Check the most recent one.

2. Your P60 — issued by your employer each April, this shows your total earnings and tax paid for the year, plus your tax code.

3. Your P45 — if you recently left a job, your former employer should have issued this.

4. HMRC’s online account — the fastest and most comprehensive way. Log in to your Personal Tax Account at GOV.UK using your Government Gateway login. You can see all your current tax codes, recent changes, and why your code is what it is. If you don’t have a Government Gateway account, you can set one up at the same link.

5. The HMRC app — available on iOS and Android, it lets you check your tax code without needing to log into a browser.

6. Call HMRC directly — 0300 200 3300, Monday to Friday 8am–6pm. Be prepared to wait.

When Your Tax Code Might Be Wrong

Tax codes are set based on the information HMRC holds. If that information is outdated or incorrect, your code will be wrong. Common situations that cause an incorrect code:

• Started a new job and your employer used an emergency code (0T or BR) because you didn’t have a P45
• Multiple jobs or pension income — HMRC may not have applied your allowance correctly across income sources
• Changed circumstances — divorced, stopped receiving a benefit in kind, paid off a student loan, changed working hours
• Claimed Marriage Allowance (or should have) — affects both partners’ codes
• State pension changes — if you’ve recently started receiving state pension, this counts as taxable income and affects your code
• Previous year underpayment — HMRC sometimes adds a reduction to your current year’s code to collect unpaid tax from a previous year, which reduces your take-home pay

How to Tell If You’re Overpaying or Underpaying

A quick sense check:

• Is your tax code lower than 1257? You may have less Personal Allowance than the standard amount — there may be a legitimate reason (benefits in kind, underpayment recovery) or it may be an error
• Is your code BR on your main job? That’s almost certainly wrong — you should have your Personal Allowance applied
• Do you have the same code on two jobs? Your Personal Allowance should only be applied once — having it duplicated means you’re underpaying tax and will face a bill

You can also use HMRC’s Income Tax Checker to see a breakdown of what tax you’re expected to pay and why, and flag any discrepancy.

How to Fix a Wrong Tax Code

Online: Log into your Personal Tax Account and use the “Tell HMRC about a change” function. HMRC will issue a new code to your employer within a few days to a few weeks.

By phone: Call HMRC on 0300 200 3300. They can usually resolve simple code queries in a single call.

Via your employer: Your HR or payroll team can apply a new code once they receive an updated P6 form from HMRC, but they cannot change the code themselves without HMRC instruction.

If you’ve been on the wrong code for a significant period:

• Overpaid tax — HMRC should adjust your code to recover the difference throughout the rest of the tax year. If you’ve overpaid across a full year, you can claim a refund via your Personal Tax Account or by calling HMRC. You don’t need to wait for them to write to you
• Underpaid tax — HMRC will usually collect smaller amounts through a coding adjustment in the following year, rather than demanding immediate payment

Real Example: How a Wrong Code Costs You

Say you started a new job in January 2026 without a P45, and your employer gave you a BR code. You’re earning £35,000 a year in that role.

• With the correct 1257L code: you pay 20% tax on £22,430 (£35,000 minus £12,570) = £4,486 per year
• With a BR code: you pay 20% on the full £35,000 = £7,000 per year

That’s £2,514 more tax per year — or nearly £210 per month more coming out of your pay than HMRC is entitled to. You’d get it back eventually, but it could sit with HMRC for months.

According to GOV.UK guidance on income tax, HMRC automatically reconciles many tax positions at the end of each year — but errors that persist unchallenged can take longer to correct and may require your intervention.

Conclusion

Your tax code is a small thing with a potentially large financial impact. The key takeaways:

• 1257L is the standard code for most UK employees in 2026 — if yours is different, understand why
• Check your code via your payslip, P60, or Personal Tax Account — it takes five minutes and can identify errors immediately
• Wrong codes are common after job changes, life events, or where multiple income sources are involved
• Overpaid tax can be claimed back — you don’t have to wait for HMRC to notice; contact them directly
• Fix errors quickly — the longer a wrong code runs, the more tax you’ve overpaid or underpaid, and the more complex the correction

Next read: Want to keep more of your income? Read our guide on claiming tax relief on pension contributions UK: /claiming-tax-relief-on-pension-contributions-uk