Wrong Tax Code? How to Fix It and Claim Your UK Refund

HMRC gets tax codes wrong more often than you'd expect. Millions of UK workers are on the wrong code at any given time, and most don't notice until they check a payslip closely — or until they wonder why their take-home pay dropped for no obvious reason.

A wrong tax code means you're either overpaying or underpaying tax. Overpaying is more common, and the good news is you can claim it back. Here's how the whole thing works.

What your tax code actually means

Your tax code tells your employer how much of your income is tax-free. The standard code for 2025/26 is 1257L, which means you get £12,570 tax-free (the Personal Allowance). Everything above that gets taxed at the usual rates.

You'll find your tax code on your payslip, your P45, your P60, and in your HMRC online account.

Signs your tax code is wrong

Look out for these:

• Your take-home pay dropped suddenly with no change in hours or salary
• Your code ends in W1, M1, or X — these are emergency tax codes, meaning HMRC is taxing you without your full allowances
• Your tax-free amount doesn't match £12,570 and you don't know why (it can differ legitimately if you have benefits-in-kind, multiple jobs, or owe tax from a previous year)
• You started a new job and your old employer's income is still being counted

Why HMRC gets it wrong

The most common causes:

• Starting a new job without giving your new employer a P45 from your old one
• Having two or more jobs where the allowance is split incorrectly
• Company benefits (like private health insurance or a company car) that change your code
• HMRC errors — they process millions of records automatically, and mistakes happen
• Leaving and rejoining the workforce mid-year

How to check and fix it

Step 1: Log into your Personal Tax Account

Go to GOV.UK and sign in. You'll see your current tax code, what HMRC thinks you earn, and what allowances they've applied. Compare this against your actual situation.

Step 2: Contact HMRC if something's off

You can do this through your Personal Tax Account online, or call the income tax helpline on 0300 200 3300 (Monday to Friday, 8am to 6pm). Have your National Insurance number and a recent payslip ready.

Tell them what's wrong. If your employment details are outdated, your income estimate is off, or you have benefits-in-kind that shouldn't be there, they'll update your record and issue a corrected code to your employer.

Step 3: Keep your details current

If you change jobs, pick up a second income, or start freelancing, make sure HMRC knows. They can't fix what they don't know about.

Claiming back overpaid tax

If you've been on the wrong code, you've probably overpaid. Here's how to get it back.

How much could you be owed? It depends on how wrong the code was and how long it was active. Being on an emergency tax code for three months on a £30,000 salary could mean you've overpaid by £500 or more. If a wrong code ran for a full tax year, it could easily be over £1,000.

For the current tax year: HMRC will usually adjust your tax code for the remaining months so you get the refund through your pay. You don't need to do anything extra once they've corrected the code.

For previous tax years: You can claim back overpaid tax for the last four tax years. Apply through your Personal Tax Account or by posting form P50 to HMRC. Refunds typically arrive within five weeks by cheque or bank transfer.

Emergency tax: the most common problem

If you start a new job without a P45, your employer will usually put you on an emergency code (often 1257L W1 or 1257L M1). This means HMRC calculates your tax month-by-month instead of cumulatively, and you lose the benefit of unused allowances from earlier months.

It's not the end of the world, but it usually means you overpay temporarily. Once HMRC receives your correct details, they'll update the code and you'll get a refund through your payroll. If it doesn't happen automatically within a couple of months, chase them.

Can your employer fix it?

No. Your employer applies whatever code HMRC tells them to. They can't change it themselves. But a good payroll department will flag if your code looks unusual and suggest you contact HMRC.

The responsibility is yours. Check your payslip regularly — it takes thirty seconds.

What to do right now

1. Log into your Personal Tax Account and check your code
2. Compare it against the standard 1257L (or whatever you'd expect given your circumstances)
3. If it's wrong, contact HMRC immediately
4. Use our salary calculator to see what your take-home should actually be on the correct code

You have four years to claim back overpaid tax. Don't leave money sitting with HMRC that belongs in your account.