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Self Assessment Tax Bill Calculator
Work out what you actually have to pay on 31 January, including the payment on account that catches most people out the first time.
2025/26 tax year · due 31 January 2027 · England, Wales & Northern Ireland
Why the January bill is bigger than the tax you owe
Most people filing for the first time budget for one number and get asked for about half as much again. The reason is that 31 January collects two separate things: the balancing payment that settles the tax year you have just filed, and the first payment on account towards the year you are currently in.
Each payment on account is half of your previous bill. So on a bill of £9,700 you pay £9,700 to settle up, plus £4,850 in advance, which is £14,550 in one go, then another £4,850 the following July. After that it evens out, but the first January is a shock if nobody warned you.
You avoid payments on account only if your last bill was under £1,000, or if more than 80% of the tax you owed was already collected at source, typically through PAYE.
What this includes
Income tax on your combined income, Class 4 National Insurance on your profit at 6% between £12,570 and £50,270 and 2% above that, and student loan repayments where they apply. Class 2 is no longer compulsory: above the small profits threshold it is treated as paid, so your record is protected at no cost.
If your profit pushes you past £100,000 your personal allowance starts to disappear and your effective rate jumps. The 60% tax trap calculator shows what that costs, and the high-earner optimiser finds the pension contribution that escapes it. For employment income, use the take-home pay calculator.
Frequently asked questions
Why is my January bill bigger than the tax I owe?
Because you usually pay two things at once: the balancing payment for the year just filed, plus the first payment on account for the year you are in. The payment on account is half of the bill you just settled.
Who has to make payments on account?
Almost everyone with a bill of £1,000 or more, unless more than 80% of your tax was already collected at source. They fall due on 31 January and 31 July.
Do I still pay Class 2 National Insurance?
It is not compulsory. Above the small profits threshold it is treated as paid, so your record is protected anyway. Below it you can pay voluntarily to keep your record complete.
Does this cover Scotland?
Not yet. Scotland sets its own income tax bands, so the figures differ. This uses England, Wales and Northern Ireland rates. Class 4 National Insurance is the same across the UK.
This is an estimate to help you budget, not a tax return. It assumes a full year of trading and no capital gains, dividends or other reliefs. Check your figures with HMRC or an accountant before you file.