Tax year the bill is for

In November 2026 the return people are finishing is often 2025/26, due 31 January 2027. The dates follow the year you pick.

How to arrive at the bill From turnover and expenses I already know the Income Tax and Class 4

Where you live England Wales Northern Ireland Scotland

Turnover

Allowable expenses

Deduction Expenses Trading allowance

Tax deducted at source

CIS deductions and similar amounts taken off this self-employed income. Not payments on account. Not PAYE from a job: if you have a job, the bands in profit mode are already wrong.

Payments on account already made towards this bill

The total of the two payments, if you made them. Leave blank if this is the first year.

Result

GOV.UK's example is a £3,000 bill with £1,800 of payments on account already made. The January total in that example is £2,700. The button loads those amounts.

Assumptions

Payments on account are not due when the relevant amount is under £1,000, or when more than 80% of the assessed Income Tax and Class 4 was deducted at source. Exactly £1,000, and exactly 80%, still mean the payments are due. That reading follows HMRC's legal manual SALF303.
Each payment is half the relevant amount. If the pence do not split evenly, the extra penny is on the first payment. Published examples divide evenly.
A positive balancing payment is added to the first payment on account, which is how GOV.UK's £3,000 example reaches £2,700. A credit is not subtracted from that payment on this page, because the published examples do not show that netting.
Class 2, student loan repayments and Capital Gains Tax are outside the relevant amount. Student loans and Capital Gains Tax can still be collected with the balancing payment.
Profit mode assumes this profit is your only income. It is the same calculation as the take-home calculator.

Result

Assumptions

Sources on GOV.UK