What BMI measures
Body Mass Index compares weight with height. It is quick, widely used and useful for population screening, but it does not separate muscle, fat, bone or fluid.
What body-fat percentage estimates
Body-fat percentage estimates how much of total body mass is fat. Tape-measure methods such as the US Navy formula infer the result from body circumferences and height. More specialised methods include skinfolds, bioelectrical impedance and DEXA scans.
Why results can disagree
A muscular person can have a higher BMI without carrying a high level of body fat. Conversely, someone can have a BMI in a conventional range while having lower muscle mass and a higher body-fat percentage.
Measurement error matters
- Tape position and tension affect circumference formulas.
- Hydration can change bioelectrical-impedance readings.
- Different formulas and devices are not directly interchangeable.
- Small week-to-week changes may be measurement noise.
Look beyond one number
Health context can include waist measurement, blood pressure, blood tests, fitness, medical history, diet and symptoms. A healthcare professional can assess factors that a simple calculator cannot.