How we build and check our calculators

What you can expect from every calculator, and how to tell us when something is wrong.

How each calculator is made

  1. Design: we choose the formula and the inputs people actually need, and keep the layout simple.
  2. Testing: results are checked against separately written reference calculations across a range of inputs, including edge cases such as zero rates.
  3. Explanation: the page explains the formula, shows a worked example and lists its sources.
  4. Review: wording is read through for accuracy and clarity before it goes live.

Sources

We prefer primary and official sources, such as government agencies and standards bodies, and link to them at the bottom of each calculator page.

Keeping pages up to date

Each page shows a last-updated date. We update the date when the calculation, the wording or the sources change. Rates, tax rules and health guidance change over time, so treat them as inputs to check, not as fixed facts.

Corrections

If you think a result or an explanation is wrong, please contact us with the inputs you used and the result you expected.

What our calculators are not

Calculators give general estimates for information and education. They are not financial, medical, legal or tax advice. See our disclaimer.

Advertising and links

TheCalcTools is free to use. If we show ads or affiliate links, they are kept separate from calculator results and do not change the numbers.

Last updated 2026-10-17.