What are tax codes? 

In simple terms, your tax code is a mix of numbers and letters that tells your employer how much Income Tax to deduct through PAYE. HMRC issues this code, not your employer.

The number reflects how much tax-free income you’re entitled to, while the letter signals your circumstances. For most people, the standard code is 1257L, which represents the £12,570 personal allowance.

You can find a full breakdown of tax codes here:

Tax codes: What your tax code means - GOV.UK

How are tax codes calculated? 

HMRC starts with your Personal Allowance and then adjusts it. For example, they may reduce it if you receive benefits (like private medical insurance) or have unpaid tax from previous years.

The process is broadly:

• Start with your tax-free Personal Allowance
• Subtract deductions (benefits, other income, underpayments)
• Convert the result into a tax code (drop the last digit and add a letter)

This sounds straightforward, but in reality, it rarely feels that way. For example:

• Full allowance → 1257L
• Reduced allowance → lower number (e.g. 1100L)
• No allowance → codes like 0T

For higher earners, things become even more complex as the tax-free allowance is gradually reduced by £1 for every £2 earned over £100,000.

Why UK tax codes feel so complicated 

The UK tax system is widely criticised for its complexity. As highlighted in industry commentary and reports, increasing changes and layered rules have made the system harder to understand and more expensive to administer.

This complexity leads to:

• Incorrect tax codes
• Over- or under-payment of tax
• Heavy reliance on individuals spotting errors themselves

It’s no surprise there are ongoing calls for simplification and reform.

How to change your tax code

If your code looks wrong, the good news is you can fix it. HMRC encourages taxpayers to use their online Personal Tax Account.

This HMRC video explains the process clearly:

How do I update my details online if my tax code is wrong? - YouTube

In short, you can sign in to your personal tax account and update this yourself. HMRC will then issue a revised tax code and notify your employer, usually within a couple of weeks.

This may come as a surprise, however the responsibility of ensuring your tax code is correct falls with you, not your employer.

So, what needs to change?

Put simply, the system needs simplifying. A better approach would include: 

• Clearer, more transparent tax code calculations 
• Fewer adjustments built into PAYE coding 
• Real-time updates based on accurate data 
• Greater automation with less reliance on taxpayer intervention

Tax codes should give clarity, not confusion. Until reform happens, understanding your code remains one of the simplest ways to stay in control of your finances.