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Tax calculator

Self Assessment Tax Calculator

Last updated: 10 July 2026Information correct for tax year: 2026/27

This calculator estimates tax on business profit plus other income. It excludes payments on account, Class 2 details, student loans, tax-code adjustments, losses, capital allowances and many reliefs.

By FinanceHub UK Editorial TeamReviewed by role: Chartered tax adviserCalculations run in your browser

Quick answer

Enter turnover, allowable expenses and other income to estimate taxable profit, Income Tax and Class 4 National Insurance.

Calculator

Enter your numbers

Enter gross business income for the year.
Enter simplified allowable expenses.
Enter other non-savings, non-dividend taxable income before allowance.
Select the applicable Income Tax region.

How to use this calculator

  1. Enter the figures that match your current scenario.
  2. Check the effective date, assumptions and any jurisdiction or plan selection.
  3. Review the breakdown, test a second scenario and verify the result before acting.

Explanation

What it is

Enter turnover, allowable expenses and other income to estimate taxable profit, Income Tax and Class 4 National Insurance.

How it works

The calculator treats positive business profit and other income as non-savings income for the regional Income Tax calculation, then applies simplified 2026/27 Class 4 rates to business profit.

When to use it

Use this tool to explore a planning scenario before checking current official rules, product documents or professional guidance.

Limitations

  • The result is an estimate based only on the inputs shown.
  • Rates, thresholds and product terms can change after the effective date.
  • The calculator does not replace an official assessment, provider quote or personalised advice.

Key terms

Estimate
A planning result produced from the stated inputs and assumptions, not a guaranteed outcome.
Effective date
The date or tax year for which a changing rule, threshold or rate has been checked.
Authoritative source
An official or regulator-backed source used to support a rule, rate or calculation method.

Formula

How we calculate this

The calculator treats positive business profit and other income as non-savings income for the regional Income Tax calculation, then applies simplified 2026/27 Class 4 rates to business profit.

Profit = turnover − allowable expenses; combined estimate = Income Tax + Class 4 NI

Statutory or methodological reference:GOV.UK official guidance — Self Assessment Tax Returns.

Formula trace: Aggregate taxable income and gains, allowances, payments on account, student-loan and class 4 NI where applicable; reconcile to HMRC calculation and tax-year rules.

Worked example

Enter realistic figures into the self assessment tax calculator and compare the result with the breakdown. Change one assumption at a time so you can see which factor has the greatest effect. Check the governing rule at GOV.UK official guidance — Self Assessment Tax Returns.

FAQ

What does the self assessment tax calculator calculate?

Enter turnover, allowable expenses and other income to estimate taxable profit, Income Tax and Class 4 National Insurance.

Which assumptions have the biggest effect?

The most important assumptions are the amounts, time period, applicable rate or threshold, and any jurisdiction or plan choice shown in the form.

How accurate is this estimate?

It is designed for planning and testing scenarios. Accuracy depends on the inputs and whether your circumstances fit the simplified method described on the page.

Can I use the result as a final decision?

No. Verify changing rules and product terms, and seek suitable professional or official guidance when the decision is material or complex.

When should I recalculate?

Recalculate after a change in income, balance, rate, term, household circumstances, tax year or official policy.

Common mistakes

  • Using a headline rate without checking whether it applies to the full amount.
  • Mixing monthly and annual figures.
  • Treating an educational estimate as an official assessment or guaranteed quote.

Tips

  • Test a cautious scenario as well as an optimistic one.
  • Keep a note of the assumptions and effective date.
  • Compare the result with official guidance or provider documents before acting.

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Sources and editorial review

Author and review

Author: FinanceHub UK Editorial Team — Editorial. Editorial policy.

Reviewed by role: Chartered tax adviser. Named qualified reviewer sign-off is pending before production.

Review record date: 2026-07-10. Next review due: 2027-03-01.