How we work: building and checking our calculators

A calculator is only useful if you can trust it. This page explains how we build our calculators, where the numbers come from and how we check that they are right.

Open formulas

Every result shows how the number comes about. The explanation below the result works through your own input step by step, with the same formula that produces the result. That way you can check every step yourself.

Sources

When a calculator rests on facts, such as a tax rate, a threshold or a conversion factor, we name the source and the date we consulted it. We prefer the original source: a government, a standards body or a scientific paper. A calculator that only does arithmetic, such as percentages or area, needs no list of sources.

Fixed checks on every publish

Every calculator has a set of fixed checks: input with a result that is fixed in advance, and invalid input that must be rejected. These often include edge cases, such as the highest and lowest allowed values. All checks run automatically every time we publish. If one fails, the calculator does not go live.

The calculation engine exists in two versions: one in your browser and one on our server. With every new version of the site we check that both agree to the last decimal. Below every calculator, under Sources and checks, you can look at some of the calculations yourself.

Last reviewed and changes

Every calculator shows when we last reviewed its content. When something substantive changes, such as a rate, a new option or a clearer explanation, it is listed with its date under Changes.

Calculated in your own browser

The calculation runs in your own browser; your input is not sent to us. If calculating in the browser fails, our server works it out instead, without storing or logging your input. The privacy page has the details.

Indicative, not advice

The results are indicative. A calculator works with the assumptions listed under Assumptions and does not know your personal situation. For an important decision about money or health, talk to a professional.

The Sumish editorial team