Self Assessment Tax Estimator 2026/27
Updated for the 2026/27tax year · Last updated
Enter your gross self-employment income and allowable business expenses to estimate your Self Assessment tax bill — income tax and Class 4 NI on taxable profit. The calculator also shows whether HMRC will require payments on account.
As a sole trader or self-employed person, you pay two taxes on your profits: income tax (the same system as employees, but paid via Self Assessment instead of PAYE) and Class 4 National Insurance contributions, which are levied specifically on self-employment profits. Unlike PAYE employees, you receive no tax deducted at source — you are responsible for setting money aside throughout the year and paying what you owe by 31 January each year.
Your taxable profit is your gross self-employment income minus all allowable business expenses. HMRC allows deductions for costs that are incurred “wholly and exclusively” for the purposes of your business. Common deductible expenses include professional subscriptions, business mileage, accountancy fees, office supplies, software subscriptions, business insurance, and a proportion of home running costs if you work from home. Claiming all legitimate expenses is essential — every pound of deductible expense reduces both your income tax and your Class 4 NI bill.
The first-time Self Assessment experience catches many sole traders off guard because of payments on account — HMRC’s system of requiring advance payments toward next year’s bill. If your qualifying tax bill (income tax plus Class 4 NI) exceeds £1,000, you will be required to make two payments in advance, each equal to 50% of the current year’s bill. These fall due on 31 January and 31 July. The January payment is particularly steep because it combines your current-year balance with the first advance payment for next year.
Estimated tax bill
£4,532
on £30,000 taxable profit
| Item | Amount |
|---|---|
| Gross income | £35,000 |
| Less: expenses | − £5,000 |
| Taxable profit | £30,000 |
| Income tax | £3,486 |
| Class 4 NI | £1,046 |
| Total tax & NI | £4,532 |
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How this calculator works
The calculator follows the Self Assessment computation as HMRC defines it for the 2026/27 tax year:
Taxable profit: Gross self-employment income minus allowable expenses. If you have additional income sources (rental income, dividends, employment income), enter the self-employment figures here — the income tax bands are applied to your total income, so your self-employment profit may fall entirely in the basic rate band or partly into the higher rate band depending on what else you earn.
Income tax: Calculated on taxable profit using the 2026/27 bands — personal allowance £12,570, basic rate 20% up to £50,270, higher rate 40% up to £125,140, additional rate 45% above. The personal allowance tapers for income above £100,000 at a rate of £1 per £2 of excess, creating an effective 60% rate in that band.
Class 4 National Insurance: Also calculated on taxable profit. For 2026/27 the rates are 6% on profits between £12,570 (Lower Profits Limit) and £50,270 (Upper Profits Limit), and 2% on profits above £50,270. Class 4 NI has no bearing on your State Pension entitlement — it is purely a revenue-raising levy on self-employment profits. Class 2 NI was effectively abolished for most self-employed people from April 2024 and is no longer a separate payment for those earning at or above the Small Profits Threshold.
Payments on account: If your combined income tax and Class 4 NI bill exceeds £1,000 and less than 80% of your total tax liability was collected at source (via PAYE or otherwise), HMRC will require payments on account. Each payment is 50% of the qualifying liability. The first is due 31 January alongside your balancing payment; the second is due 31 July. You can apply to reduce payments on account via your HMRC online account if you expect your income to fall next year.