How to estimate your self employed tax bill in the UK
Estimating your bill is not about predicting HMRC to the penny — it is about knowing whether you are in the right ballpark before you spend the cash. Here is a practical order of operations for the 2026/27 tax year.
1. Fix your profit number first
Start with profit you expect for the tax year (6 April to 5 April), not money sitting in your bank. Pull a year-to-date figure from your bookkeeping, add known pipeline work, then subtract costs you are confident are allowable. If you are unsure on a borderline item, model a conservative and an optimistic version — the gap between them is your risk range.
2. Run Income Tax and Class 4 NI together
For most sole traders, the big two are Income Tax on taxable profit and Class 4 National Insurance on the same profit (above the lower profits limit). Personal Allowance taper above £100,000 can bite harder than people expect, so if you are near that zone include it in any estimate rather than assuming the full £12,570 always applies.
3. Add the things this stage usually misses
Student loan repayments, pension contributions, Gift Aid, and other income all change the final return. If any of those apply, treat your first estimate as a baseline and adjust manually, or use a take-home calculator that exposes those fields once you have stable numbers.
4. Turn the annual figure into cash flow
A good annual estimate still surprises people in January because HMRC may want payments on account in July as well. Once you have an annual liability, divide by twelve for a monthly sweep or use a weekly set-aside percentage so your savings rhythm matches how you actually get paid.
Run the numbers
Use these tools in order: profit → full tax stack → optional refinements.
If you have PAYE salary as well, use the optional other-income field on the self-employed tax calculator so bands stack correctly.
5. Sanity-check against last year
If your business shape is stable, last year’s return is the best calibration tool you have. If this year’s estimate is wildly higher or lower, ask why before you trust it — a single large invoice, a change in expenses, or a forgotten PAYE job can explain most swings.
6. Write down your assumptions
Keep a one-line note with your estimate: profit figure used, date checked, and whether you included PAYE or dividends elsewhere. Future-you will not remember why you trusted a number, and your accountant will not guess. Good estimates are as much about traceability as about maths — especially if HMRC asks questions later.