About SumLark

Many calculators online are hard to trust. They do not say where their numbers come from, they use last year’s rates, or they bury the answer under adverts and sign-up forms. We wanted calculators that show their working and that you can check.

Rates, thresholds and rules come from official UK sources such as GOV.UK, HMRC and Ofgem. Each calculator lists its sources at the bottom of the page, with what we took from each one. We do not invent figures and we do not copy them from other calculators.

Before a calculator is published, we test it against an official tool or an official worked example wherever one exists: for example HMRC’s own tools and tables, and the GOV.UK holiday and redundancy calculators. Where there is no official tool we say so on the page and show how the figure is worked out step by step.

Tax years start on 6 April, the minimum wage changes each April and the energy price cap changes every three months. We update the calculators when the official figures change, and every page shows the date it was last updated. We re-check the calculators against the official sources on a regular schedule.

SumLark is independent. We are not part of, or endorsed by, HMRC, GOV.UK, Ofgem or any company. The calculators are free to use with no sign-up. We hope to pay for the site with advertising in the future. Advertisers and sponsors can never change a calculation or a result.

Found a mistake, or want a calculator we do not have yet? Email us at [email protected]. See also our Contact page.