BMI Calculator
Calculate body mass index from weight and height in metric or imperial units, see the WHO adult category, and the weight range that corresponds to a normal BMI.
Formula
- body mass index, in kg/m²
- body mass, in kilograms
- standing height, in metres
How it works
Body mass index divides body mass by the square of height. The WHO adult classification calls a value below 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight (pre-obese), and 30 or more obese, with classes I to III from 30, 35 and 40.
BMI is a screening measure of weight for height in populations. It does not distinguish fat from muscle or show where fat is carried, it is interpreted differently for children against age- and sex-specific percentiles, and lower cut-offs have been proposed for some Asian populations. A category is a prompt for further assessment, not a diagnosis.
Worked example
An adult weighs 70 kg and is 175 cm tall.
- Height = 1.75 m; 1.75² = 3.0625.
- BMI = 70 / 3.0625 = 22.86 kg/m².
The BMI is 22.9 kg/m², in the WHO normal range. A BMI of 18.5 to 24.9 at this height corresponds to 56.7 to 76.3 kg.
These are the values the calculator opens with, so you can check its output against this example.
Assumptions
- An adult (18 years or older) who is not pregnant.
- Accurate measured weight and height rather than self-reported values, which tend to overstate height and understate weight.
Common mistakes
- Using the adult categories for a child or adolescent.
- Entering height in centimetres where metres are expected. The unit selector handles both, and feet and inches.
- Reading a BMI as a measure of body fat or of health on its own.