What BMI is
Body mass index divides weight in kilograms by height in metres squared. It was designed for population statistics in the 1830s and remains useful for exactly that: at a population level it tracks health risk reasonably well.
Where it breaks down
BMI does not know what you are made of. Muscle is denser than fat, so a 90 kg lifter and a 90 kg sedentary person of the same height get the same score and very different health pictures. It also ignores where fat sits; visceral fat around the organs matters far more than fat on the hips.
Ethnic differences matter too: several health bodies recommend lower overweight thresholds (23 rather than 25) for people of South Asian, Chinese and Japanese descent because metabolic risk rises at lower BMIs.
Better numbers for people who train
- Waist-to-height ratio: keep waist below half your height.
- Weight trend, not weigh-ins: a 7-day smoothed average removes water noise. Vitalume shows this by default.
- Strength progression: if lifts climb while the trend falls, you are recomping; BMI will never show it.
- Body-fat estimates: tape measurements or a consistent smart-scale reading, tracked over months rather than days.