If your tax code is wrong

This site is an independent UK tax-code explainer. We explain the legal mechanisms, but we do not have access to your data or provide personal tax advice.

Is it wrong, or is it out of date?

It is important to understand how to tell whether your code is actually wrong. A tax code is a mechanical instruction from HMRC sent directly to your employer’s payroll software. If the software follows that exact instruction, the employer has processed the payroll correctly. The code itself is only “wrong” if the instruction does not reflect your current life circumstances.

Most wrong codes are, in fact, entirely right for the out-of-date information HMRC currently holds. The calculation is a pure product of the data on your file. If the data is outdated, the resulting code will be outdated.

The rules governing this are found in reg 14(1). HMRC determines the code based on a strict list. They must have regard to: reliefs the employee is entitled to; other PAYE income; tax overpaid in earlier years not yet repaid; tax remaining unpaid from earlier years; over-repaid tax; other non-PAYE income unless the employee objects; and “such other adjustments as may be necessary”. If any of these elements change and HMRC is unaware, the code will not match your reality.

How to change your tax code

The route to correcting the code goes directly through HMRC. You cannot ask your employer to change the code on your behalf. The employer is legally bound to use the instruction HMRC issues. They cannot alter it, even if you provide evidence that your circumstances have changed.

The employer cannot intervene in this dispute. The employer is legally shielded from liability as long as they operate the code supplied by HMRC. Therefore, sending evidence to your payroll department is entirely ineffective. They lack the legal authority to override the reg 14(1) instruction, even if the error is obvious to all parties.

You must update your underlying information so HMRC can issue a revised reg 14(1) instruction. The most secure and direct route is through HMRC’s authenticated Personal Tax Account. Once the data is updated, HMRC will automatically generate a new code and transmit it to your employer.

What it costs, which is nothing

No fee is needed to update your information or to have a new code issued. This independent site takes no fee, and the government service is entirely free. The reader should not send personal details to anyone promising a refund in exchange for a cut of the money. There is an industry of companies offering to claim refunds for a fee, but you can manage the update yourself directly with HMRC.

What happens on the next payday

We cannot promise a refund or a specific outcome. Adjusting your code ensures your tax is calculated on the correct basis going forward. Once HMRC issues the corrected instruction, your employer will apply it on the next available payday. If the code operates on the normal cumulative basis, the payroll software will look at your total pay and total tax for the entire year to date. It applies your newly corrected allowance to the whole period and subtracts the tax you have already paid. The deduction adjusts automatically. If too much tax was taken under the out-of-date code, the excess is reduced or repaid directly through the payroll mechanism.

Why the payslip can be right and wrong at once

It is important to remember that the payroll software operates automatically based on these instructions. The software does not know your personal circumstances; it only knows the data HMRC has transmitted. If the data transmitted is outdated, the deduction will be mathematically accurate according to the instruction, but factually wrong for your life. This is why maintaining accurate records with the authorities is the only way to ensure the system operates as intended.

The only place a person’s actual code can be viewed is HMRC’s authenticated Personal Tax Account. This site cannot know it, and does not try to. You must take action directly with HMRC to establish the correct instruction.

Before deciding a code is wrong it is worth reading it properly: how to check your tax code walks through the parts of the code, what to compare them against, and which of those checks this site can do for you and which only HMRC can. If the code changed rather than looked wrong from the start, why your tax code changed lists the reg 14(1) factors that move it.

Common questions