Sources & Methodology
Last updated August 1, 2026
Fueld's health and fitness metrics are estimates derived from established, publicly available formulas and research. They are for general informational and educational purposes only and are not medical advice. Below are the sources and methods behind each calculation.
Body Mass Index (BMI)
BMI is calculated as weight (kg) divided by height (m) squared, following the standard definition used by the U.S. Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO).
- CDC — About Adult BMI: https://www.cdc.gov/bmi/adult-calculator/
- WHO — Body mass index: https://www.who.int/data/gho/data/themes/topics/topic-details/GHO/body-mass-index
VO₂ Max
When available, VO₂ Max is read directly from Apple Health, which estimates it from your Apple Watch heart-rate and workout data. Where Fueld estimates cardiorespiratory fitness, it uses established equations from the American College of Sports Medicine (ACSM) and the Cooper Institute.
- ACSM's Guidelines for Exercise Testing and Prescription (American College of Sports Medicine)
- Apple — Estimating VO₂ Max with Apple Watch: https://support.apple.com/en-us/HT208925
Fitness Age
Fitness Age is an estimate that compares your cardiorespiratory fitness and related health inputs (such as VO₂ Max, resting heart rate, body composition, and activity) to population norms for your age and sex. The approach is based on peer-reviewed cardiorespiratory-fitness research.