Income Tax and National Insurance explained (with real examples)
Most people's first real encounter with Income Tax and National Insurance is a payslip that doesn't match the number they were quoted at interview. Here's what's actually happening to that money, and why.
How Income Tax bands work
UK Income Tax isn't a flat percentage of your whole salary — it's applied in bands. You get a tax-free Personal Allowance on the first slice of income, then progressively higher rates apply only to the income above each band threshold, not to your entire salary. This is why earning slightly more than a band threshold never actually leaves you worse off overall — only the portion above the line is taxed at the higher rate.
One quirk catches higher earners out: the Personal Allowance itself starts shrinking once income passes a set threshold, tapering away gradually rather than disappearing all at once — which is why very high earners can face a noticeably higher effective rate on that slice of income than the headline band would suggest.
What National Insurance actually pays for
National Insurance (NI) is a separate deduction from Income Tax, calculated on earnings above its own threshold. Unlike general taxation, NI contributions specifically build your entitlement to the State Pension and certain contributory benefits — it's less a tax on income in the abstract and more a record of contributions tied to state support later in life. Employees pay Class 1 NI; the self-employed pay differently (see our self-employed tax guide).
Your tax code — something like 1257L — tells your employer how much tax-free Personal Allowance to apply before deducting tax from each payslip. It changes when your circumstances do, such as taking on a second job or receiving a benefit-in-kind.
Why two people on the same salary can take home different amounts
Pension contributions, student loan repayments, benefits-in-kind (like a company car), and salary sacrifice schemes all change the taxable figure your Income Tax and NI are calculated on — which is why "£35,000 a year" doesn't map to one single take-home number. It depends on what's deducted before tax, what's deducted after, and which tax code HMRC has issued.
Checking your own numbers
Because band thresholds and rates are reviewed at the start of each tax year, the safest approach is to calculate against current figures rather than remembering last year's numbers — a small band shift can change a result more than people expect.
A quick note
Tax bands, thresholds and rates change from one tax year to the next — always check current gov.uk figures before making decisions based on an exact number. See our full disclaimer.