What BMI is and how it is calculated
Body mass index (BMI) is weight in kilograms divided by the square of height in metres: BMI = weight / (height in metres)^2. For a 70 kg person who is 1.75 m tall, that is 70 / (1.75 x 1.75) = 22.9 kg/m2. The measure was devised by the Belgian statistician Adolphe Quetelet in the 1830s and adopted by the WHO as a simple way to classify weight across large populations.
The World Health Organization divides adults into four bands: underweight below 18.5, healthy weight from 18.5 to 24.9, overweight from 25 to 29.9, and obese at 30 and above. These cut-offs come from associations between BMI and health risk observed across large groups, not from any single individual's body.
Why athletes should read BMI with caution
BMI uses total body weight and cannot tell muscle from fat. Muscle is denser than fat, so a lean, muscular athlete carries more weight for their height and can be flagged overweight or even obese while holding very low body fat. Rowers, sprinters, rugby players and many strength athletes routinely sit above 25 with single-digit fat percentages.
The opposite error happens too: someone with little muscle and excess fat can sit inside the healthy band while carrying unhealthy fat, a pattern sometimes called normal-weight obesity. BMI is best treated as a rough, free screen that flags who might benefit from a closer look, not as a measure of body composition or fitness.
Better measures of body composition
If you want to know how much of your weight is fat versus lean tissue, BMI is the wrong tool. Waist circumference and waist-to-height ratio capture central fat, which carries the most health risk, and need only a tape measure. Skinfold calipers, bioelectrical impedance scales, and DEXA scans estimate fat percentage directly, with DEXA being the most accurate of the everyday options.
For most endurance athletes, trends matter more than any single number. Tracking weight, waist and how your clothes and training feel over weeks tells you more than one BMI reading. Use BMI to place yourself roughly, then lean on these richer measures if the number surprises you.
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, which falls in the overweight band, though a muscular athlete at that figure may carry little excess fat.
Frequently asked questions
Is BMI accurate for athletes?
Often not. BMI counts all weight as mass and cannot separate muscle from fat. Because muscle is dense, lean, muscular athletes frequently read as overweight or obese despite low body fat. For athletes, body-fat measures such as skinfolds, bioelectrical impedance or a DEXA scan give a far truer picture than BMI alone.
What is a healthy BMI?
The World Health Organization defines a healthy adult BMI as 18.5 to 24.9 kg/m2. Below 18.5 is classed as underweight, 25 to 29.9 as overweight, and 30 or above as obese. These ranges describe population health risk and should be read alongside waist measurement, body composition and overall health, not in isolation.
How do I calculate BMI?
Divide your weight in kilograms by your height in metres squared. For example, 70 kg and 1.75 m gives 70 / (1.75 x 1.75) = 22.9. If you use centimetres, divide height by 100 first. The same formula applies to adults of all ages; children use age and sex specific percentile charts instead.
Does BMI measure body fat?
No. BMI only relates total weight to height; it does not measure fat directly. Two people with the same BMI can have very different body-fat levels depending on how much muscle they carry. To assess fat, use waist circumference, waist-to-height ratio, skinfolds, impedance scales 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.