How this calculator works
Tax years: 2026/27, 2022/23 and 2021/22 (6 April to 5 April). This annual-threshold estimate assumes one employment, a full year of self-employment and ordinary working-age rates. In 2026/27 employee Class 1 is 8% between GBP 12,570 and GBP 50,270, then 2%; Class 4 is 6% and 2% over those profit thresholds, and compulsory Class 2 is zero. Historical years include compulsory Class 2 where due. The 2022/23 Class 1 result uses HMRC director annual rates, not an exact reconstruction of monthly payslips. If both income sources are present, the total is before any combined-class annual-maximum adjustment, so HMRC may charge less.
Formula
Class 1 = main rate x max(0, min(earnings, upper threshold) - lower threshold) + upper rate x max(0, earnings - upper threshold). Class 4 uses the corresponding profit rates. Total estimate = Class 1 + compulsory Class 2 + unadjusted Class 4.
Worked example
In 2026/27, GBP 30,000 employment earnings give (30,000 - 12,570) x 8% = GBP 1,394.40 Class 1. GBP 30,000 self-employment profit with no employment gives GBP 1,045.80 Class 4 and no compulsory Class 2.
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Frequently asked questions
Why might my payslips differ?
Most employees pay by earnings period, using rounded weekly or monthly thresholds. Bonuses, multiple jobs and in-year changes can make their annual total differ from this annual-basis estimate.
Can I find income from contributions?
Yes. Choose an income mode and enter the total contribution estimate and the other income. A zero contribution cannot identify a unique income below the threshold, and historical Class 2 jumps leave some targets unattainable.
Is employer National Insurance included?
No. These are contributions paid by the worker. Employer contributions, voluntary contributions, reduced-rate categories, pension-age rules and refunds are excluded.
What happens when I am both employed and self-employed?
Both components are shown before HMRC applies combined-class limits. Do not treat their sum as a final Self Assessment liability. The inverse modes use this same unadjusted model.