· health, bmi
BMI: What It Measures, What It Misses
How BMI is calculated, what the categories mean, and why athletes and older adults often get misleading numbers.
Body Mass Index divides your weight in kilograms by the square of your height in meters (kg/m²). It was designed in the 19th century as a population statistic — and it remains useful for exactly that.
Standard WHO categories
- Below 18.5 — underweight
- 18.5–24.9 — healthy range
- 25.0–29.9 — overweight
- 30.0 and above — obese
Where BMI misleads
- Muscular people: muscle is denser than fat, so athletes can read 'overweight' while very lean.
- Older adults: body composition shifts with age; the same BMI can mean different health risk.
- Ethnic variation: some populations face elevated health risks at lower BMIs than others.
Treat BMI as a first-pass screening number, not a diagnosis. Pair it with waist circumference or a body-fat estimate for a fuller picture — our BMI calculator shows your category plus the healthy weight range for your height.