How We Calculate

A calculator is only useful if you can trust the number it gives you. This page explains how we choose formulas, how we test every calculator and how we keep content accurate.

1. Sources

We base every calculator on a published, widely accepted method:

• Health: the U.S. Centers for Disease Control and Prevention (CDC), the National Institutes of Health (NIH), the World Health Organization (WHO) and peer-reviewed studies. For example, our BMI calculator uses the CDC formula and adult categories.
• Money: standard financial formulas (such as the amortization formula for loans) and official rates from government agencies.
• Units and dates: internationally defined conversion factors (for example, 1 inch = 2.54 cm exactly) and the Gregorian calendar.

The main sources for each calculator are listed at the bottom of its page.

2. Separate, testable formulas

The math behind each calculator lives in its own small program, separate from the page design. That makes it possible to test the formula on its own with known inputs and answers.

3. Automated tests

Every calculator has a set of test cases with known results, including typical values, edge cases and invalid input. The tests run automatically every time anything on the site changes. If a single result moves, even by a cent or a tenth of a point, the change cannot go live until the difference is explained and fixed.

4. Worked examples match the calculator

The examples on each page are produced with the calculator itself and are included in its tests. If you enter the same numbers, you get the same result.

5. Clear limits

When a formula is an estimate, we say so. When a result depends on personal factors the calculator cannot see, we explain them and point to a professional where appropriate.

6. Updates

Pages show the date they were last updated. When official rates or guidelines change, we update the calculator, its examples and its tests together.

Editorial independence

We do not accept payment to change results, rankings or recommendations, and we do not publish sponsored content disguised as editorial content. If advertising appears on the site, it is clearly separated from the calculators and articles.

Corrections

Spotted something wrong? Contact us. Confirmed errors are corrected promptly and the page’s update date is refreshed.