Health
BMI Calculator
Enter height and weight. The formula is shown, and so are its limits.
The formula
BMI = weight in kg ÷ (height in metres)²
height in metres = height in cm ÷ 100
Worked example
Someone 175 cm tall weighing 74 kg
- 175 cm = 1.75 m
- 1.75² = 3.0625
- 74 ÷ 3.0625 = 24.2
Where this goes wrong
Reading a category as a diagnosis
BMI is a ratio of weight to height squared and nothing else. It cannot distinguish muscle from fat, so a 100 kg athlete at 180 cm scores 30.9 and lands in the same band as someone with a very different body; it does not account for build, age, sex or where fat is carried, and it was devised in the 1830s to describe populations rather than to assess individuals. A number of 24.2 and a number of 25.2 differ by less than the error in a bathroom scale and a morning meal, yet they fall either side of a line. Use it as one coarse indicator among several, and take an actual health question to someone qualified to answer it.
Questions
- What are the BMI categories?
- The World Health Organization classification for adults is: under 18.5 underweight, 18.5 to 24.9 normal range, 25 to 29.9 overweight, 30 and above obese. These are conventional cut-offs applied to populations, not thresholds where anything changes about a person — someone at 24.9 and someone at 25.1 are indistinguishable.
- Is BMI accurate for athletes?
- No, and this is its best-known failure. Muscle is denser than fat, so a well-trained athlete can register as overweight or obese while carrying very little fat. The formula has no way to see the difference — it only knows total mass and height.
- Does BMI work for children?
- Not in this form. Children are assessed against age-and-sex percentile charts rather than the fixed adult cut-offs, because body composition changes throughout growth. This page uses the adult formula and should not be applied to a child.
- What should I use instead?
- Waist circumference and waist-to-height ratio are widely used alongside BMI because they say something about where fat is carried, which BMI cannot. But no single number decides anything: the useful version of this question goes to a clinician who can look at more than one measurement.