Definition
BMI (Body Mass Index) is a simple indicator that relates a person's weight to their height. It's calculated by dividing weight in kilograms by height in meters squared. Used by the World Health Organization (WHO), it gives a quick read on body size and the risks linked to being overweight or underweight.
Detailed explanation
The BMI formula
The math is simple: BMI = weight (kg) / height (m)²
For example, someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 / (1.75 × 1.75) = 22.9.
How to interpret the results
The WHO defines the following categories for adults:
- Less than 18.5: underweight
- 18.5 to 24.9: normal weight
- 25 to 29.9: overweight
- 30 to 34.9: class I obesity (moderate)
- 35 to 39.9: class II obesity (severe)
- 40 and above: class III obesity (morbid)
These thresholds come from epidemiological studies linking BMI to the risk of cardiovascular disease, type 2 diabetes, hypertension and other health issues.
The limits of BMI
Despite its usefulness in public health, BMI has serious limits at the individual level. It doesn't tell fat mass from muscle mass apart. A muscular athlete can have a BMI of 28 (overweight category) while carrying very little body fat and being in excellent health.
BMI also doesn't account for fat distribution. Yet abdominal (visceral) fat is far more dangerous than subcutaneous fat stored around the hips or thighs. Waist circumference is a more relevant complementary measure here.
BMI also isn't suited to children, older adults, pregnant women, or some ethnic groups whose risk thresholds differ.
In practice
Calculate your BMI in seconds with the BMI calculator. Use the result as a starting point, not a final verdict. If your BMI sits outside the normal range, that's a signal to dig deeper with other measurements.
For a more complete read on your body composition, combine BMI with waist circumference (high risk above 94 cm for men, 80 cm for women) and, if possible, a body fat measurement via bioelectrical impedance or skinfold calipers.
If you're aiming for a healthy weight, focus on a moderate calorie deficit rather than BMI as a numeric target. The trend and your habits matter more than any single number.
Useful links
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