Body mass index from height and weight
Body mass index and its WHO category, with the formula.
How BMI is calculated
Body mass index is weight in kilograms divided by height in metres squared. With a height of 175 cm and a weight of 70 kg: 70 ÷ (1.75 × 1.75) = 22.9. The formula was devised in the 19th century and is still used by the WHO as a quick population-level guide — precisely because it needs nothing but scales and height.
Category boundaries
For adults the WHO uses these boundaries: below 16 — severe underweight, 16–18.5 — underweight, 18.5–25 — normal, 25–30 — overweight, 30–35 — class I obesity, 35–40 — class II, above 40 — class III. The boundaries are the same for men and women; the difference in interpretation appears only in childhood.
What BMI does not show
The index does not distinguish muscle from fat: athletes often score high, while people with little muscle and hidden fat score normal. It does not account for age, fat distribution (belly fat is more dangerous than fat on the hips), pregnancy or fluid retention. So the category is a reason to talk to a doctor, not a diagnosis, and certainly not a reason to start a strict diet based on a single number.
Frequently asked questions
Does BMI work for athletes?
With a caveat: muscle is denser than fat, so people with a lot of muscle often score as overweight on BMI without carrying excess fat.
What does the category mean?
It is a WHO population reference, not a diagnosis. Only a doctor can assess your health, taking age, build, blood pressure and test results into account.
What BMI is considered normal?
For adults, 18.5 to 24.9. Below 18.5 is underweight, 25–29.9 is overweight and 30 or above is obesity.
Checked: 26 October 2026. For reference only — the result depends on the material and the skill of the installer.