Calculators Percentage

How we build and check our calculators

Accuracy is the whole point of a calculator. This page explains exactly how we make sure the numbers are right.

1. Standard, published formulas

We use the formulas taught in finance, math and health textbooks and published by official bodies – for example the standard amortization formula for loans, the compound interest formula used by the SEC's Investor.gov, the BMI formula and categories used by the CDC and WHO, and the Mifflin-St Jeor equation for calorie needs. Each calculator page shows its formula.

2. Official sources on every page

Each calculator lists the government agencies or standards bodies behind its rules under “Sources & references”. Country tax calculators also show the specific tax authority page and the date the rules were last updated.

3. Automated testing

Before any change is published, an automated test suite runs every calculator with known inputs and compares the output with hand-checked answers (for example, a BMI of 24.7 for 80 kg at 180 cm). A change that breaks a result is not published.

4. Worked examples you can verify

Every guide includes a worked example with the exact inputs and outputs, so you can repeat the calculation yourself and confirm it.

5. Reviews and updates

Tax brackets, contribution limits and benefit amounts change every year. We review these values when the responsible agency publishes new figures and show a “last updated” date on each page. If you spot an outdated value, please contact us.

Official references we rely on

6. Limits of any calculator

Results are estimates based on the inputs you provide and simplified rules. Real loans, taxes, benefits and health outcomes depend on details a calculator cannot see. Use results as a starting point and confirm important decisions with a qualified professional. See our terms.