What do tax codes mean in the UK and how do they work?

Understanding tax codes is essential for ensuring the correct amount of tax is deducted through payroll. They determine how much Income Tax is deducted from pay, and even small errors can lead to incorrect deductions over time.

Tax codes are comprised of different numbers and letters that all mean different things, and they are also all applied differently in practice.

For example, the most common tax code in the UK is 1257L. It’s used for most people with one job and no untaxed income, unpaid tax or taxable benefits. 

What is a tax code?

A tax code is the instruction HMRC sends employers to tell them how much  Income Tax  to deduct from each pay packet. It's the calculation key behind the  PAYE system : it applies the right  tax-free Personal Allowance , factors in any benefits or untaxed income, and decides the employee's final take-home pay.

It ensures that employees receive the correct tax-free Personal Allowance and pay the right amount of tax through PAYE .

When hiring a new employee, you can usually find their tax code on their P45 form or the Starter Checklist.

👉 To note: HMRC calculates each employee’s tax code based on their income, benefits and personal circumstances.

What do the numbers in a tax code mean?

The numbers in a tax code show the employee's  net tax-free allowance : how much they can earn each year before paying Income Tax. HMRC starts with the full allowances the employee is entitled to, then subtracts adjustments such as a company car or unpaid tax from previous years. The final digit is dropped to create the code. So  1257  means a  £12,570 allowance . A lower number means taxable benefits or prior tax owed are reducing that threshold.

📌 Example: Tax code 1257L means £12,570 tax-free allowance 

 👉 To note: Adjustments may be made to this number if the employee has untaxed income or benefits, such as a company car.

What do the letters in a tax code mean?

The letters in a tax code act as  status flags  for payroll software. They tell the system which tax category the employee falls into: whether they get the  full Personal Allowance , are taxed at a higher rate because of a second job, or live in  Scotland or Wales  and need devolved tax rates applied.

Common UK tax code letters

A full list of tax code letters is available on the GOV.UK website .

What is the most common tax code?

The most common code is 1257L , the UK’s default baseline for employees with a single income and no taxable benefits. In 2026, this code is particularly significant due to the ongoing freeze of the £12,570 Personal Allowance ; because this threshold hasn't risen with inflation since 2021, fiscal drag is quietly pulling more workers into higher tax brackets. The OBR forecasts an extra 4 million workers will pay basic rate and 3 million higher rate by 2028/29.

It means:

• £12,570 tax-free allowance 

• Standard Income Tax rates applied to remaining income

What does a K tax code mean?

A K tax code means an employee has taxable income that exceeds their Personal Allowance , so additional tax must be collected through payroll.

This usually happens when the value of untaxed income or benefits is greater than the employee’s tax-free allowance.

When is a K tax code used?

A K tax code may apply if an employee has:

• Benefits-in-kind (e.g. company car, private medical insurance) 

• Unpaid tax from previous years 

• Multiple income sources not fully taxed at source 

This results in increased Income Tax deductions. However, employers must ensure that a K tax code does not deduct more than 50% of an employee’s gross pay in any single pay period to protect their take-home pay.

⚠️ Warning: even when an employee has a K code, payroll must never deduct more than 50% of their gross pay in a single pay period (Regulation 7, Income Tax (PAYE) Regulations 2003).

📌 Example of a K tax code:  Tax code K500 means £5,000 is added to the employee’s taxable income instead of being tax-free.

How are tax codes calculated?

Tax codes are calculated by HMRC based on an employee’s total income and personal circumstances.

HMRC considers:

• Personal Allowance 

• Benefits-in-kind (e.g. company car, private medical insurance). These are reported via P11D forms or payrolled benefits , which directly trigger HMRC to adjust the tax code.

• Untaxed income 

• Previous underpaid or overpaid tax 

👉 To note: Benefits-in-kind can reduce the Personal Allowance, which lowers the tax code number.

Why might a tax code change?

Tax codes aren't fixed. HMRC updates them throughout the year whenever an employee's circumstances change, so deductions stay accurate and no surprise bill lands at year-end. A mid-year  pay rise , a new  company car  or a  second income  can all trigger a new code.

Common reasons include:

• Starting a new job 

• Having more than one job 

• Receiving taxable benefits 

• Changes in income 

• Corrections to previous tax calculations

What is an emergency tax code?

An emergency tax code acts as a temporary "holding" status applied when HMRC lacks the real-time data, such as a P45 from a previous employer or a completed Starter Checklist, needed to determine a person's correct tax-free allowance.

Common emergency codes include:

• 1257L W1 (Week 1): Tax is calculated only on the income for that specific week.

• 1257L M1 (Month 1): Tax is calculated only on the income for that specific month.

• 1257L X: A general marker used by payroll software to indicate a non-cumulative calculation is in place.

Because these codes are non-cumulative , they ignore any tax you’ve already paid or allowances you’ve used earlier in the tax year. This often results in a "flat" tax deduction that doesn't account for overpayments, which is why employees may see a sudden shift in their net pay once HMRC receives their full employment history and issues a permanent code.

How can you check if a tax code is correct?

The fastest way to check a tax code is to compare what's on the employee's payslip with their HMRC Personal Tax Account or the HMRC app . Mistakes do happen, especially around job changes or after receiving a new benefit, so it's worth checking every few payslips. Spotting a wrong code early, for example a BR (Basic Rate) code applied to a main job by mistake, can prevent a large underpayment bill at year-end.

Employees can find their active tax code on:

• Payslips:  the quickest way to see the code currently in use.

• P60  or P45 forms:  useful at year-end or when changing jobs.

• HMRC Personal Tax Account or app:  shows the income sources HMRC is currently tracking and the active tax code.

What happens if a tax code is wrong?

If a tax code is incorrect, it can result in employees paying too much or too little tax.

To correct an incorrect tax code, contact HMRC directly through the HMRC app or your Personal Tax Account . HMRC will then issue a P6 or P9 coding notice to your employer.

Once HMRC adjusts your code, they notify your employer digitally. If you’ve overpaid , you’ll typically receive an automatic refund through your next payslip as a "tax credit," increasing your take-home pay immediately.

If you’ve underpaid , HMRC usually reduces your tax-free allowance for the rest of the year to collect the debt in small increments from your future wages. If the tax year has already ended, they will send a P800 calculation letter explaining how to claim your refund via bank transfer or pay any balance online.

How do tax codes affect payroll?

Tax codes directly affect payroll calculations by determining how much Income Tax is deducted from employee pay.

For employers, this means:

• Applying the correct tax code in payroll systems 

• Updating codes promptly upon receiving HMRC P6 or P9 notifications through the Full Payment Submission (FPS) or the HMRC portal.

• Ensuring accurate PAYE deductions 

Using payroll software can help reduce errors and ensure tax codes are applied correctly.