What Is Income Tax in the UK and How Does the Tax Code Work?

Introduction to UK Income Tax

Income tax is the main direct tax paid by individuals in the United Kingdom. It is collected by HMRC (His Majesty's Revenue and Customs) on most types of income you receive, including wages, self-employment profits, most pension income, rental income, and certain investment income. Understanding how it works — and how your tax code determines how much is deducted from your pay — is fundamental to understanding your financial position.

Income Tax Rates and Bands (England, Wales, and Northern Ireland)

Income tax is not a flat rate — it's charged at progressive rates, meaning you pay higher rates only on income above each threshold. For 2025/26:

• Personal Allowance: £12,570 — income up to this amount is tax-free
• Basic Rate: 20% — on income between £12,570 and £50,270
• Higher Rate: 40% — on income between £50,270 and £125,140
• Additional Rate: 45% — on income above £125,140

Scotland has separate income tax rates and bands, set by the Scottish Parliament, with more bands and slightly different thresholds — if you live in Scotland, your income tax position may differ from someone earning the same amount elsewhere in the UK.

How the Tax Bands Work in Practice

A common misconception is that if you earn £55,000, you pay 40% tax on all of it. That's not how it works. Tax is charged progressively:

• First £12,570: £0 tax
• Next £37,700 (£12,571 to £50,270): 20% = £7,540
• Remaining £4,730 (£50,271 to £55,000): 40% = £1,892
• Total tax: £9,432
• Effective tax rate: 17.1% of total income

This progressive structure means the "marginal rate" (what you pay on the next pound you earn) differs from your "effective rate" (total tax as a percentage of total income). Many people confuse these two figures.

The Personal Allowance Tapering

For higher earners, the personal allowance gradually reduces. For every £2 of income above £100,000, the personal allowance reduces by £1. This creates an effective marginal tax rate of 60% for income between £100,000 and £125,140 — one of the peculiarities of the UK tax system that makes pension contributions particularly valuable in that income range.

What Is a Tax Code?

Your tax code is a combination of letters and numbers used by your employer (and pension provider) to calculate how much income tax to deduct from each pay packet. It tells your employer how much tax-free income you're entitled to in the tax year.

The most common tax code is 1257L, which applies to most employees with one job and the standard personal allowance. The number 1257 represents one-tenth of the personal allowance (£12,570 ÷ 10 = £1,257), and the letter L indicates the standard personal allowance.

Other common code letters include:

• M: Marriage Allowance transferred from a partner
• N: Marriage Allowance transferred to a partner
• T: Personal allowance requires adjustment (other calculations needed)
• 0T: No personal allowance (used when starting a new job without a P45)
• BR: Basic rate applied to all income (used for second jobs or pension income)
• D0: Higher rate applied to all income
• NT: No tax deducted (rare, specific circumstances)
• K: Negative allowance — you owe tax from a previous year or have taxable benefits in kind that exceed your personal allowance

Why Your Tax Code Might Be Wrong

HMRC estimates that hundreds of thousands of UK workers are on incorrect tax codes at any given time, resulting in either overpaying or underpaying tax. Common reasons include:

• Starting a new job without a P45 (causing a 0T emergency code)
• Having multiple jobs or income sources
• Company benefits in kind (company car, private medical insurance) added incorrectly
• Marriage Allowance not applied
• Overpaid tax from a previous year incorrectly carried forward

Check your tax code on your payslip or through your personal tax account at gov.uk. If it seems wrong, contact HMRC on 0300 200 3300 or update your details online. Overpaid tax is refunded; underpaid tax is typically collected through an adjustment to the following year's code.

PAYE: Pay As You Earn

Most employed people pay income tax through PAYE — their employer deducts tax and National Insurance from each pay packet and pays it to HMRC on their behalf. The employee never sees the gross amount; they simply receive their net (after-tax) pay.

PAYE works on a cumulative basis throughout the tax year. If you underpay in early months (perhaps because you started a new job partway through the year), the system catches up and takes more later. If you overpay, HMRC typically issues a refund after the tax year ends, or the amount is carried forward.

Self Assessment

If you're self-employed, have income from property rental, earn above £100,000, have income from savings or investments above the relevant allowances, or have other untaxed income, you'll need to complete a Self Assessment tax return each year. The deadline for online returns is 31 January following the end of the tax year.

Self Assessment calculates your total income from all sources, applies the relevant allowances and reliefs, and determines your total tax liability. You pay any tax owed (and the first payment on account for the following year) by 31 January.

Key Tax-Free Allowances

Beyond the personal allowance, several other allowances can reduce your income tax bill:

• Marriage Allowance: Transfer £1,260 of personal allowance from a non-taxpayer to a basic-rate taxpayer partner — worth £252 per year
• Savings Allowance: Basic-rate taxpayers can earn £1,000 in interest tax-free; higher-rate taxpayers £500
• Dividend Allowance: £500 of dividend income tax-free (2025/26)
• Trading Allowance: £1,000 of self-employment or side-hustle income tax-free
• Property Allowance: £1,000 of rental income tax-free

Conclusion

Understanding income tax and your tax code puts you in control of your own financial affairs. Checking your tax code regularly, claiming all allowances you're entitled to, and understanding the rates that apply to your income level can prevent overpaying and ensure you're not caught out by unexpected bills. Use your personal tax account at gov.uk to view and manage your tax position, and don't hesitate to contact HMRC if something doesn't look right.