A short definition
Body mass index (BMI) is a person’s weight in kilograms divided by the square of their height in metres. It was devised in the 19th century by the Belgian mathematician Adolphe Quetelet and became the standard screening measure for weight in adults in the late 20th century, because it is quick, free and needs no special equipment.
Which BMI calculator should I use?
- BMI Calculator — the main tool for adults, in kg, pounds or stones.
- BMI Calculator for Women and BMI Calculator for Men — the same maths, with notes on body fat and waist size for each sex.
- Child BMI Calculator — BMI-for-age percentiles for ages 2 to 19.
- BMI Calculator by Age — picks adult categories or children’s percentiles from your age.
- BMI Calculator kg — a simple metric version.
- BMI Calculator UK — stones and pounds with the ranges used in UK guidance.
- BMI Calculator India — Asian BMI ranges.
- TyG-BMI Calculator — combines BMI with fasting blood tests.
- Pregnancy Weight Gain Calculator — uses pre-pregnancy BMI during pregnancy.
Adult BMI ranges
| BMI (kg/m²) | Adult category (WHO) |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40 or more | Obesity class III |
Lower thresholds (23 for overweight, 27.5 or 25 for obesity) are used for people of Asian and some other backgrounds, who face higher health risks at a given BMI.
The limits of BMI
BMI does not distinguish fat from muscle, does not show where fat is stored and was developed mainly from data on adults of European descent. It is best used as a first check, together with waist measurement, blood pressure and blood tests. Pregnant women, very muscular people, older adults and some ethnic groups need extra care when interpreting it.
How our calculators are built
Adult results use WHO, NICE and Asia-Pacific cut-offs. Children’s results use the published CDC 2000 BMI-for-age reference with the LMS method, calculated for age in months. All the numbers on these pages — including the healthy-weight tables — are generated from the same formulas the calculators use, and nothing you enter is stored.