What BMI is and how it is calculated
Body mass index (BMI) is weight in kilograms divided by height in meters squared: 70 kg at 1.75 m is 70 / (1.75 x 1.75) = 22.9 kg/m2. Adolphe Quetelet devised it in the 1830s and the WHO adopted it for population screening.
WHO adult bands: underweight below 18.5, healthy 18.5 to 24.9, overweight 25 to 29.9, obese 30 and above. They describe risk across groups, not one body.
Why athletes should read BMI with caution
BMI cannot tell muscle from fat. Muscle is denser, so a lean, muscular athlete can read overweight or obese at very low body fat. Rowers, sprinters and strength athletes often sit above 25 with single-digit body fat. The reverse error, excess fat inside the healthy band, is normal-weight obesity.
Better measures of body composition
Waist circumference and waist-to-height ratio capture central fat, the riskiest kind, with a tape measure. Skinfold calipers, bioelectrical impedance scales and DEXA scans estimate fat percentage directly. DEXA is the most accurate. Track trends, not single readings.
Worked example
A 175 cm person weighing 70 kg checks their BMI:
| Weight | 70 kg |
| Height | 175 cm (1.75 m) |
| Formula | 70 / (1.75 x 1.75) |
| BMI | 22.9 kg/m2 |
| WHO category | Healthy weight |
At 90 kg and 180 cm the same calculation gives 27.8, the overweight band, though a muscular athlete may carry little fat at that figure.
Frequently asked questions
Is BMI accurate for athletes?
BMI is often inaccurate for athletes. It cannot separate muscle from fat, so lean, muscular athletes often read as overweight or obese despite low body fat. Skinfolds, impedance or DEXA give a truer picture.
What is a healthy BMI?
A healthy adult BMI is 18.5 to 24.9 kg/m2 by the World Health Organization definition. Below 18.5 is underweight, 25 to 29.9 overweight, and 30 or above obese. Read it alongside waist and body composition.
How do I calculate BMI?
Divide weight in kilograms by height in meters squared: 70 kg and 1.75 m gives 70 / (1.75 x 1.75) = 22.9. Convert centimeters to meters first. Children use age- and sex-specific percentile charts.
Does BMI measure body fat?
No. BMI only relates total weight to height. Two people with the same BMI can have very different body-fat levels depending on muscle. To assess fat, use waist circumference, skinfolds, impedance or a DEXA scan.
Sources
- WHO (2000). "Obesity: preventing and managing the global epidemic." WHO Technical Report Series 894. Link
- Quetelet (1832). Adolphe Quetelet's index of weight over height squared, the origin of modern BMI.