Self Assessment Tax Calculator

Estimate your UK Self Assessment bill as a sole trader: income tax, Class 4 National Insurance and payments on account.

Self-employed profit	£45,000
Other income taxed through PAYE	£0
Tax already paid through PAYE	£0
Last year's Self Assessment bill (for payments on account)	£6,000
Scottish taxpayer	No

Self-employed profit

Other income taxed through PAYE

Tax already paid through PAYE

Last year's Self Assessment bill (for payments on account)

Scottish taxpayer

Tax and NI for the year

£8,431.80

Income tax £6,486.00 and Class 4 NI £1,945.80 (6% between £12,570 and £50,270, 2% above), less £0 already paid. Payments on account of £4,215.90 each are due on 31 January and 31 July for next year. The balancing payment is the bill minus any payments on account already made (£3,000 × 2 if based on last year).

Income tax: £6,486.00
Class 4 NI: £1,945.80
Each payment on account: £4,215.90
Balancing payment: £2,431.80

From tax due to your bill

Income tax£6,486
Class 4 NI+£1,946
Bill for the year£8,432

About the Self Assessment Tax Calculator

Self-employed profits are taxed at the usual income tax rates (2026/27: 20% basic, 40% higher, 45% additional above £125,140 in England, Wales and Northern Ireland; Scotland has its own bands) plus Class 4 NI at 6% of profits between £12,570 and £50,270 and 2% above. Class 2 NI is no longer compulsory. If your bill is over £1,000 and less than 80% was taxed at source, you also make two advance payments on account for next year, each half of this year's bill, due 31 January and 31 July.

The formula

Bill = income tax + Class 4 NI − tax paid at source; each POA = 50% of the bill

Worked example

Example: Tax and NI for the year = £9,731.80

Inputs: Self-employed profit: £50,000; Other income taxed through PAYE: £0; Tax already paid through PAYE: £0; Last year's Self Assessment bill (for payments on account): £6,600; Scottish taxpayer: No.
Formula: Bill = income tax + Class 4 NI − tax paid at source; each POA = 50% of the bill
Result: Tax and NI for the year = £9,731.80
Income tax: £7,486.00
Class 4 NI: £2,245.80
Each payment on account: £4,865.90

Questions people ask

When is the deadline?

Online returns and payment by 31 January after the tax year.

Can I reduce payments on account?

Yes, if you expect lower profits, but interest applies if you reduce too far.

Sources

The rules and rates this calculator uses come from the official pages below. Its results are estimates, not an official determination, and rules change, so check the latest figures there before you act on them.