The formula, and the one thing it can't see
Weight divided by height squared. In US units it is weight (lb) × 703 ÷ height (in)2. The number is a screening shortcut, not a body scan: it uses only your mass and height, so it counts muscle and fat the same. A lean athlete and a sedentary person of the same weight and height get the identical BMI.
WHO categories for adults
| Category | BMI range | What it flags |
|---|---|---|
| Underweight | Below 18.5 | Possible nutritional shortfall, weaker bones |
| Normal | 18.5 – 24.9 | Lowest weight-related risk |
| Overweight | 25.0 – 29.9 | Raised risk of heart disease, type 2 diabetes |
| Obese, class I | 30.0 – 34.9 | Significant health risk |
| Obese, class II | 35.0 – 39.9 | High risk |
| Obese, class III | 40.0 and up | Very high risk |
The healthy band maps to a weight range for your height: at 1.75 m that is 56.7 to 76.2 kg. The bands are population averages, so treat a result near a boundary as a prompt to look closer, not a verdict.
Where BMI is read differently
- Children and teens. Same formula, different reading. Body composition changes with age, so paediatric BMI is scored as an age-and-sex percentile, not against the fixed adult bands.
- Asian populations. The WHO suggests a lower overweight cutoff, sometimes around 23, because metabolic risk rises at a lower BMI.
- Older adults. After about 65 a slightly higher BMI (25–27) often tracks with better outcomes, so the standard bands are less useful.
Common questions
How is BMI calculated?
Divide your weight in kilograms by your height in metres squared. For 70 kg at 1.75 m: 70 divided by (1.75 x 1.75 = 3.0625) is a BMI of 22.9. In pounds and inches, multiply weight by 703 and divide by height in inches squared.
What is a healthy BMI?
The WHO healthy range for adults is 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or more is obese. The ranges are the same for men and women.
Why does BMI say I am overweight when I am muscular?
BMI only uses weight and height, so it cannot tell muscle from fat. Muscle is denser than fat, so an athlete can land in the overweight or obese band while carrying very little body fat. For a lean, trained body use a body fat percentage or waist measurement instead.
