UK Self-Assessment First-Year Bill Calculator (payments on account)
uk_first_year_self_assessmentYour real first-January Self Assessment bill — the year’s tax PLUS 50% of next year’s, due the same day — with the exact dated payment schedule. Computes a UK sole trader’s 2025-26 Self Assessment bill (income tax stacked on top of any PAYE income, plus Class 4 National Insurance) and then the part general AI reliably misses: payments on account. First-time filers owe 150% of their bill on 31 January 2027 — the full year’s tax plus the first half of next year’s, in one payment, for income earned up to ~22 months earlier. The tool applies the exact boundary tests (POAs are waived when the bill is under £1,000 or when more than 80% of your tax was collected at source through PAYE), the post-April-2025 late-payment interest formula (Bank rate + 4%, currently 7.75% — models still quote the old + 2.5%), and flags whether Making Tax Digital’s quarterly reporting catches you from April 2026.
Input Schema
| Name | Required | Description | Default |
|---|---|---|---|
| profit | No | Self-employment profit for 2025-26 (£) Tax year 6 Apr 2025 – 5 Apr 2026: revenue minus allowable expenses (your taxable profit, not turnover). | |
| firstYear | No | Is this your FIRST Self Assessment year? First-timers get the 150% shock: the whole year’s bill plus the first payment on account land on the same day. Returning filers have already part-paid via last year’s payments on account. | yes |
| priorBill | No | Last year’s total Self Assessment bill (£) Only used when this is NOT your first year: it set the two payments on account (50% each) you have already made toward this year. | |
| payeIncome | No | Employment (PAYE) income in the same year (£) Salary taxed through payroll. It uses up your personal allowance and basic-rate band BEFORE your profit — and because its tax is collected at source, it feeds the 80% test that can spare you payments on account entirely. |