Self-employed tax guide
UK Self Assessment Tax Bill: Income, Expenses and Payments on Account
Self Assessment brings together income that PAYE may not fully tax, including self-employment profit. The amount due can include Income Tax, Class 4 National Insurance and payments on account.
Updated 21 September 2026 · 9 min read · Educational guidance
Turnover is not taxable profit
Turnover is the income received by the business. Taxable profit broadly starts after allowable business expenses are deducted, subject to the detailed rules and any capital allowances.
Entering turnover as self-employment profit overstates the calculator result. Private spending and amounts already included elsewhere should not be deducted twice.
Example: A sole trader with £48,000 of turnover and £13,000 of allowable business expenses starts with £35,000 of profit, not £48,000.
Related: Estimate a UK tax refund →
What the bill can contain
Income Tax is calculated after allowances and reliefs. Class 4 National Insurance can apply to self-employment profit. Student-loan repayments can also be collected through Self Assessment.
Payments on account are advance instalments toward the following year. They affect cash due at the deadline but do not mean the same income is taxed twice.
- ✓ Income Tax
- ✓ Class 4 National Insurance
- ✓ Student or postgraduate loan repayment
- ✓ First payment on account
- ✓ Any balancing payment from the previous year
Related: Pension tax relief explained →
Records that make the estimate stronger
Reconcile invoices, bank transactions and receipts rather than guessing a round expense total. Separate business and private use and keep the evidence for the required period.
If you also have PAYE income, enter the gross pay and PAYE Income Tax deducted so the calculator can reconcile both parts of the year.
Related: Student-loan repayments →
Frequently asked questions
Do I enter turnover or profit?
Enter taxable profit after eligible business expenses.
Are payments on account another tax?
No. They are advance payments toward the next Self Assessment bill.
Does the tool calculate every business rule?
No. Capital allowances, losses, basis periods and complex reliefs require a fuller calculation.
Official sources and important note
General educational information only, not personal tax advice. Confirm your position with HMRC or a qualified UK tax professional.