How the UK Tax System Actually Works: A Plain-English Guide

The Personal Allowance

Every UK resident has a Personal Allowance — the amount of income you can earn before paying any income tax. For 2025/26 it's £12,570. This figure has been frozen since 2021, which in real terms means more people are paying more tax as wages rise with inflation.

If your income exceeds £100,000, the Personal Allowance tapers away — you lose £1 of allowance for every £2 earned above £100,000. Between £100,000 and £125,140, your effective marginal income tax rate is 60%.

Income tax bands

Above the Personal Allowance, income tax operates in bands:

Band	Income	Rate
Basic rate	£12,571–£50,270	20%
Higher rate	£50,271–£125,140	40%
Additional rate	Over £125,140	45%

These are marginal rates — you pay 20% only on the income within that band, not on everything you earn.

National Insurance

National Insurance (NI) is a separate deduction that funds state benefits and the State Pension. The rates for employees in 2025/26:

• 8% on earnings between £12,570 and £50,270
• 2% on earnings above £50,270

NI is calculated on a weekly or monthly basis, not annually — which matters if your income fluctuates.

Your tax code

Your tax code (usually something like 1257L) tells your employer how much of your income to treat as tax-free. If it's wrong — which happens more often than you'd expect — you can end up overpaying or underpaying tax.

You can check and update your tax code through HMRC's online service. Reclaiming overpaid tax is straightforward once you know the process.

The takeaway

Understanding these mechanics doesn't require an accountant. It requires knowing where to look and what the numbers mean. A payslip that previously looked like a black box starts to make sense.

For education and entertainment only. Tax rules change — always verify current rates with HMRC or an independent adviser.