Methodology: how we build and check our tools

Every VitalCalc tool follows the same process so you can trust the answer — and check it yourself.

1. Recognized formulas and sources

Formulas come from published, widely accepted sources. The main ones used on this site are:

• World Health Organization BMI categories and waist-to-hip cut-offs
• Mifflin-St Jeor equation for BMR (1990)
• US Navy circumference method for body fat
• Boer (1984) lean body mass and Mosteller (1987) body surface area formulas
• Institute of Medicine (2009) pregnancy weight gain guidelines
• Standard obstetric dating (Naegele's rule, 280 days from the last period)

2. Tested against worked examples

Before a tool goes live, it is run with the example inputs shown on its page and the result is compared with a hand-checked answer. Edge cases — zero, very large numbers, leap years, empty fields — are tested too, and the tool shows a clear message instead of a wrong number.

3. Shown on every page

Each page has a How it works section, the formula and a worked example, so you can repeat the calculation by hand.

4. Rounding

Calculations use full precision internally. Results are rounded only for display — money to cents, most measurements to 2–4 decimal places.

5. Limits

Tools give estimates for general information. They cannot know your full situation. For money, health, legal or tax decisions, confirm with a qualified professional.

6. Updates and corrections

Rates and tables that change (such as tax brackets) are reviewed at least once a year. If you think a result is wrong, please tell us — we check every report.