UK Tax Codes Explained: What They Mean and How to Check Yours

All tax calculations in this guide use 2025/26 tax year rates and thresholds.

Your tax code determines how much income tax you pay each month. Get it wrong, and you could be overpaying by hundreds or even thousands of pounds. Here's how to understand what your tax code actually means and whether it's correct.

What Is a Tax Code?

A tax code is a combination of letters and numbers that tells your employer how much tax to deduct from your salary through PAYE (Pay As You Earn). It represents your tax-free allowances and any adjustments HMRC has made based on your circumstances.

You'll find your tax code on your payslip, P60, or P45. If you've never noticed it before, check your latest payslip. It's usually near the top, alongside your National Insurance number.

Breaking Down a Tax Code

The most common tax code is 1257L. Here's what each part means:

• 1257: The numbers show your personal allowance with the final zero removed. So 1257 represents a personal allowance of £12,570.
• L: The letter indicates which category of taxpayer you are. L means you're entitled to the standard personal allowance.

Your employer multiplies the number by 10 to calculate your annual tax-free amount, then spreads this across your pay periods. On 1257L, you can earn £12,570 before paying any income tax.

Common Tax Code Letters

The letter in your tax code tells your employer which rules to apply. Here are the ones you're most likely to encounter:

Understanding K Codes

A K code is special because it means you owe more tax than your personal allowance covers. This typically happens when:

• You have significant taxable benefits (company car, private medical insurance)
• You're repaying tax underpaid in previous years
• You owe tax on state pension

With a K code, instead of receiving tax-free income, additional tax is collected. For example, K475 means HMRC is collecting extra tax as if you had a negative allowance of £4,750. However, there's a built-in protection: HMRC cannot deduct more than 50% of your gross pay through a K code adjustment.

Emergency Tax Codes

If you see W1, M1, or X after your code (like 1257L W1), you're on an emergency tax code. This means:

• W1 (Week 1): Tax calculated weekly, ignoring previous weeks
• M1 (Month 1): Tax calculated monthly, ignoring previous months
• X: Non-cumulative basis

Emergency codes are applied when your employer doesn't have your complete tax history. This usually happens when you start a new job without a P45 from your previous employer. The problem is that these codes don't account for allowances you've already used, potentially leading to overpayment.

Why Your Tax Code Matters

Your tax code directly controls your take-home pay. A wrong code doesn't just mean administrative hassle. It means real money leaving your account that shouldn't.

The Cost of Getting It Wrong

The good news: if you've overpaid tax due to a wrong code, you can claim it back. But you'll need to spot the error first, as HMRC doesn't always catch these automatically.

Tax Code Errors Are Common

Many taxpayers find themselves on incorrect codes at any given time. Common causes include:

• Employers reporting benefits late or incorrectly
• Starting a new job without providing a P45
• Changes in circumstances (like returning a company car) not being communicated
• HMRC estimates being wrong
• Marriage Allowance claims not processed correctly
• Multiple income sources causing confusion

How to Check If Your Tax Code Is Correct

Checking your tax code takes just a few minutes and could save you significant money. Here's how to do it:

Step 1: Find Your Current Tax Code

Your tax code appears on:

• Your payslip (usually near the top)
• Your P60 (end of year tax summary)
• Your Personal Tax Account on gov.uk
• HMRC app

Step 2: Understand What It Should Be

For most employees with one job and no complications, the standard code should be 1257L. If your code is different, there should be a clear reason.

Ask yourself:

• Do I receive any taxable benefits from my employer?
• Do I have more than one job or pension?
• Did I underpay tax in previous years?
• Do I claim Marriage Allowance?
• Did I recently start this job?

Step 3: Verify with a Calculator

Use our tax calculator to model what your take-home pay should be with your current code versus the standard code. If there's a significant unexplained difference, your code may need reviewing.

Step 4: Check Your Personal Tax Account

Your HMRC Personal Tax Account shows exactly what adjustments are included in your tax code. Log in at gov.uk/personal-tax-account and review:

• Benefits and expenses included
• Any underpayments being collected
• Estimated income figures
• Other adjustments

If anything looks wrong, such as a benefit you no longer receive or an income estimate that's wildly off, you can request a correction directly through your account.

When and How to Change Your Tax Code

You should review your tax code whenever your circumstances change. Common triggers include:

When to Request a Change

• Starting a new job: Ensure your employer has your P45 to avoid emergency tax
• Getting a company car: Or giving one back
• Receiving private medical insurance: Or losing this benefit
• Starting or stopping a second job
• Getting married or registering a civil partnership: Marriage Allowance may apply
• Your income changing significantly: Especially above or below £100,000
• Starting to receive state pension
• Noticing an error: Any time you spot something wrong

How to Request a Change

You can't set your own tax code directly. HMRC issues codes to employers based on the information they hold. To request a change:

When you request a change, HMRC reviews the information and, if they agree, issues a new tax code to your employer. This typically takes 2-4 weeks, though urgent corrections can be faster.

The Starter Checklist Explained

If you don't have a P45 when starting a new job, your employer will ask you to complete a Starter Checklist. The statement you choose determines your initial tax code:

• Statement A: This is your first job since 6 April and you haven't received benefits. You'll get 1257L on a cumulative basis.
• Statement B: This is your only job now, but you've had another since 6 April. You'll get 1257L on Week 1/Month 1 (emergency) basis.
• Statement C: You have another job or receive a pension. You'll get BR (basic rate, no allowance).

Choosing the wrong statement leads to an incorrect tax code, so read each option carefully.

What to Do If You've Overpaid Tax

If you've been on the wrong tax code and overpaid, you're entitled to a refund. How you claim depends on when the overpayment happened:

During the Current Tax Year

Once HMRC corrects your code, your employer should automatically adjust your tax to account for any overpayment earlier in the year. This happens through the cumulative system. You'll see a larger-than-usual take-home pay in the months following the correction.

From Previous Tax Years

If you've overpaid tax in previous years, you may receive a P800 tax calculation from HMRC after the year ends. This shows whether you're owed a refund or owe additional tax.

If you don't receive a P800 but believe you've overpaid, you can:

• Check your Personal Tax Account for previous years
• Contact HMRC to request a review
• Complete form R40 (for those not in Self Assessment)

You can claim refunds for the last 4 tax years. Beyond that, overpayments are generally lost.

The £100k Personal Allowance Trap

One of the most significant tax code changes happens when your adjusted net income exceeds £100,000. Your personal allowance is gradually withdrawn at a rate of £1 for every £2 earned above the threshold, until it disappears entirely at £125,140.

This creates an effective 60% marginal tax rate in that band. Your tax code reflects this by showing a reduced allowance (e.g., 757T instead of 1257L) or eventually 0T.

If you're near the £100k threshold, proactive management is essential. Pension contributions can reduce your adjusted net income and potentially restore your full allowance. For a complete breakdown, see our £100k tax trap guide.

Special Cases

Multiple Jobs

If you have more than one job, you'll have a different tax code for each. Your personal allowance should only be applied to one job, usually the higher-paying one. Other jobs typically use BR (basic rate with no allowance).

If your main job changes or income levels shift, you may need to update which job gets your allowance. Check your Personal Tax Account to see how your codes are currently allocated, or contact HMRC if you're unsure which job should receive your personal allowance.

Self-Employment and Employment

If you're employed and self-employed, your employment income is taxed through PAYE using your tax code, whilst self-employment income goes through Self Assessment. Your tax code may be adjusted to collect estimated tax on your self-employment income throughout the year.

Scotland and Wales

Scottish residents have different income tax rates and bands, indicated by an S prefix (e.g., S1257L). Welsh residents have a C prefix, though currently Welsh rates match England and Northern Ireland.

Your tax code prefix is based on where you live, not where you work. If you've recently moved between countries within the UK, check your Personal Tax Account to ensure your code has been updated.

How Savin Hood Monitors Your Tax Code

Staying on top of your tax code manually means checking payslips regularly and understanding complex HMRC letters. For busy professionals, especially those with variable income, bonuses, or RSUs, it's easy for errors to slip through.

The Savin Hood income tracker helps you stay on top of your tax position automatically:

Payslip Tax Code Tracking

When you upload or log your payslips, Savin Hood extracts your tax code and compares it against what you should be on based on your circumstances. The tracker identifies:

• Unexpected code changes: Alerts you if your tax code changes without explanation
• Wrong code detection: Flags if you appear to be on BR, 0T, or an emergency code when you shouldn't be
• Code vs plan mismatch: Checks whether your current code fits your income plan for the year

If your tax code doesn't match your expected position, you'll receive an alert explaining what's wrong and what action to take.

Tax Bill and Overpayment Warnings

Based on your year-to-date income and tax code, Savin Hood projects your end-of-year position. You'll receive warnings if you're heading towards an unexpected tax bill, overpaying and due a refund, or need to take action while there's still time in the tax year.

This is particularly valuable if you have variable income. A tax code set for a £90,000 salary won't be right if you're actually earning £110,000 with bonuses. By the time HMRC catches up, you could face an unexpected bill.

Threshold Monitoring

Your tax code can be affected by crossing key thresholds. The tracker monitors your income and alerts you when you're approaching:

• £100,000: Personal allowance starts tapering (changes your tax code)
• £125,140: Personal allowance fully withdrawn (code goes to 0T or similar)
• £100,000 childcare cliff: Either parent exceeding £100,000 loses Tax-Free Childcare and extended free hours

Getting an alert before you cross a threshold gives you time to act, whether that's increasing pension contributions or timing bonus receipt.

Instead of discovering a tax code problem on your March payslip (when it's too late for the current tax year), you'll know months ahead and can take action.

Frequently Asked Questions

What does the L in my tax code mean?

The L suffix means you're entitled to the standard personal allowance. Most employees have a tax code ending in L, such as 1257L, which indicates a personal allowance of £12,570.

Why has my tax code changed?

Your tax code can change for several reasons: receiving taxable benefits (company car, private medical insurance), owing tax from a previous year, changes to your income level, starting a second job, or receiving state pension. HMRC automatically updates your code based on information from employers and benefit providers.

What is an emergency tax code?

An emergency tax code (often 1257L M1 or 1257L W1) is applied when your employer doesn't have your full tax history, typically when you start a new job. It calculates tax based on that pay period only, which can result in overpaying. Monitor your payslips and contact HMRC if the code persists beyond your second or third pay period.

How do I know if my tax code is wrong?

Signs your tax code might be wrong: you're paying significantly more or less tax than expected, you've changed jobs and tax seems high, you're still on an emergency code after several months, or your circumstances changed (e.g., stopped receiving a benefit) but your code hasn't. Use HMRC's online service or a tax calculator to verify.

Can I change my tax code myself?

You can't directly change your tax code, but you can ask HMRC to review it by contacting them online, by phone (0300 200 3300), or by updating your details through your Personal Tax Account. HMRC will then issue a new code to your employer if a change is needed.

Key Takeaways

• Your tax code determines how much tax is deducted from your pay. Wrong codes cost real money
• The standard code 1257L represents the £12,570 personal allowance
• Letters indicate your category (L = standard allowance, BR = all at basic rate, K = you owe extra)
• Emergency codes (W1, M1) are temporary and can cause overpayment if not corrected
• Check your payslip and Personal Tax Account regularly to spot errors early
• Request changes through HMRC's online services, app, or phone line, not your employer
• Above £100,000 income, your personal allowance tapers, changing your tax code
• You can claim refunds for the last 4 tax years if you've overpaid
• Track your income with Savin Hood to get alerts before threshold issues arise

Official Resources

• Gov.uk: Tax codes
• HMRC: Check your Income Tax
• Gov.uk: Personal Tax Account
• Gov.uk: Claim a tax refund

Disclaimer

This article is for general information only and is not financial, tax, or legal advice. Rules can change and your circumstances matter. If you're unsure, consider speaking to a regulated adviser or accountant.