About this calculator and its methodology

Checked against official guidance: 26 October 2026.

Thinking Bytes publishes this independent calculator. It is not HMRC, an accountant or personal tax advice. Contact Thinking Bytes by email to report an error.

Calculation method

Trading profit is gross relevant trading income less the chosen eligible deduction, capped at zero for this estimate. Additional Income Tax equals tax on salary plus profit minus tax on salary alone. Class 4 NI is calculated separately on self-employed profit. Results are rounded to whole pounds for planning.

Coverage and limitations

Rates cover 2024/25, 2025/26 and 2026/27, with separate Scottish non-savings Income Tax bands. The estimate assumes standard tax bands, UK residence and liability to Class 4 NI. It does not model State Pension age exemptions, Class 1/Class 4 annual maximum adjustments, pensions, student loans, tax-code adjustments, savings, dividends, benefits, VAT, capital gains, CIS credits, loss relief or payments already made. Losses are floored at zero, not used to claim relief.

Sources and verification

Rates, guidance and worked examples were checked on 26 October 2026. Rate data is shared by the static content and browser calculator. Future changes require explicit verification, rather than silently guessing future rates.

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