What it is and what it is for
Body mass index relates your weight to your height with a simple formula: weight in kilograms divided by height in metres squared. The World Health Organization uses it as a first reference to classify adult weight as underweight, normal weight, overweight or obese.
It is a screening tool, not a diagnosis. It does not tell muscle from fat, and it ignores age, sex and where fat is stored. Use it as a starting point and, if you have concerns about your weight, talk to a healthcare professional.
How to use it, step by step
- Type your current weight in kilograms.
- Type your height in centimetres, without converting to metres.
- Press Calculate to get your BMI to one decimal place.
- Read the category in brackets: underweight below 18.5, normal up to 24.9, overweight up to 29.9 and obese from 30.
- Repeat when your weight changes to follow the trend, always using the same scale.
Worked example
A person of 70 kg and 175 cm has a BMI of 22.9, which is normal weight. At 90 kg and 170 cm the BMI is 31.1, which falls in the obese range of the WHO classification. Two people with the same BMI can have very different body compositions.
Frequently asked questions
Does it work for children and teenagers?
No. Under 18 it is read against percentile charts for age and sex, not these ranges.
Why can an athlete show up as overweight?
Because muscle weighs more than fat and BMI cannot tell them apart. In those cases waist size and body fat percentage matter more.
Is it the same for every population?
Not exactly. Some bodies propose lower thresholds for Asian populations, for example.
Does it replace a medical appointment?
No. It is an indicative reference and gives no medical advice or diagnosis.