Self Assessment tax estimator
A sole trader's Self Assessment bill is income tax on trading profit above the £12,570 personal allowance, plus Class 4 National Insurance, under HMRC's Income Tax rates and Self-employed National Insurance rates. Profit up to £50,270 is taxed at 20% income tax and 6% Class 4 National Insurance. This calculator estimates the combined bill and how it splits.
How is my Self Assessment tax worked out?
Two charges sit on trading profit. The first is income tax: the first £12,570 is covered by the personal allowance, then 20% up to £50,270, 40% up to £125,140 and 45% above, under HMRC's Income Tax rates. The second is Class 4 National Insurance, charged at 6% on profit between £12,570 and £50,270 and 2% above that, per gov.uk's Self-employed National Insurance rates. Adding them together gives the estimated Self Assessment bill.
Class 2 National Insurance used to be a separate weekly charge. From 2024/25 it is no longer payable if profit is at or above the Small Profits Threshold. That year is still treated as paid towards the State Pension, but nothing is added to the bill. That is why the Class 2 line above reads £0.
What would I pay on a worked example?
On a £35,000 profit, income tax is about £4,486: nothing on the first £12,570, then 20% on the £22,430 above it. Class 4 NI is charged on that same slice, about £1,346. The combined bill is an estimated £5,832, an effective 16.7% of the profit. If the profit rose above £50,270, the part above would be taxed at 40% income tax and 2% Class 4.
Common questions
How is my Self Assessment tax worked out?
Your bill is income tax on your profit (after the £12,570 personal allowance, then 20%, 40% and 45% bands) plus Class 4 National Insurance. On a £35,000 profit that comes to about £5,832, of which £4,486 is income tax and £1,346 is Class 4 NI.
Do I still pay Class 2 National Insurance?
Not as a separate charge. From 2024/25 a self-employed person with profit at or above the Small Profits Threshold is treated as having paid Class 2, so nothing is due but the year still counts towards the State Pension. If your profit is below that threshold you can choose to pay Class 2 voluntarily to protect that entitlement.
What counts as profit?
Profit is your business income minus allowable expenses, not your turnover. Put the profit figure in above, not your total sales. Tax and National Insurance are charged on the profit.
When is the tax due?
For the 2026/27 tax year the return and the balancing payment are due by 31 January 2028. If your bill is over £1,000 you will usually also make payments on account, which the payments on account explainer covers.
Does this include my employment income?
No. This assumes the profit is your only taxable income for the year. If you also have a salary taxed under PAYE, the two are added together for income tax, which pushes more of the profit into higher bands.
What would I pay on a worked example?
On a £35,000 profit, income tax is about £4,486: nothing on the first £12,570, then 20% on the £22,430 above it. Class 4 NI is charged on that same slice, about £1,346. The combined bill is an estimated £5,832, an effective 16.7% of the profit.
Sources & Rate Verification
Rates verified against official statutory guidance on 6 September 2026.