BMI calculation

How is BMI calculated?

Body mass index is calculated by dividing weight, in kilograms, by the square of height, in metres: BMI = weight / height². Example: for 70 kg and 1.75 m, BMI is 70 ÷ (1.75 × 1.75), about 22.9.

The WHO categories

These are the World Health Organization thresholds for adults aged 18 to 65.

The limits of BMI

BMI is a population indicator, quick and simple, but it does not directly measure body fat.

• It does not distinguish muscle from fat: a very muscular athlete can be classed as overweight while in excellent health.
• It ignores fat distribution. Abdominal fat is the most closely linked to cardiovascular risk, so waist circumference is a useful complement. In adults, a waist above about 94 cm (37 in) for men and 80 cm (31.5 in) for women is often used as a warning sign.
• It is interpreted differently in older people, for whom a slightly higher BMI is not necessarily unfavourable, and for some ethnic backgrounds, for which lower thresholds are sometimes recommended.

Children and teenagers

For under-18s, BMI is read on growth charts by age and sex, such as those used by paediatricians. The adult thresholds in the table do not apply.

What BMI does not replace

This tool gives a general indication, not a diagnosis. For any question about your weight or health, talk to a doctor, who will consider your whole situation.

Frequently asked questions

Is BMI reliable?

It is a general indicator that does not account for muscle mass or fat distribution. It is useful as a first look, but does not replace a health professional's advice.

Is my data sent anywhere?

No. The calculation happens in your browser; your weight and height are neither transmitted nor stored.

Does BMI apply to children?

Not with the same thresholds. For children and teenagers, it is read on growth charts by age and sex.

What is the ideal BMI?

The WHO considers 18.5 to 24.9 the normal range. Within that range there is no single ideal value.