Quick answer: BMI is weight in kilograms divided by height in meters squared. For adults, the World Health Organization lists 18.5 to 24.9 as normal weight, 25 to 29.9 as overweight and 30 or more as obesity. It is a screening number, not a diagnosis, and this calculator stores nothing.
How to use the BMI Calculator
- Choose metric (kg and cm), US imperial (lb, feet and inches) or UK (stones, pounds, feet and inches).
- Enter your weight and height.
- Read your BMI, the category and the weight range that corresponds to a BMI of 18.5 to 24.9 for your height.
How BMI is calculated
BMI is your weight in kilograms divided by your height in meters squared. In imperial units it is 703 × weight in pounds ÷ height in inches squared. For example, 70 kg at 170 cm gives 70 ÷ (1.70 × 1.70) = 24.2, which is in the normal range.
BMI categories for adults
These are the World Health Organization categories for adults aged 20 and over:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity (class I) |
| 35.0 – 39.9 | Obesity (class II) |
| 40.0 and above | Obesity (class III) |
What BMI cannot tell you
- It is a screening number, not a diagnosis. It does not measure body fat, where fat is stored, fitness or overall health.
- Muscle counts as weight. Athletes and very muscular people can have a high BMI with low body fat.
- Age, sex and ethnicity matter. Older adults and people who have lost muscle can have a normal BMI with high body fat. Some health bodies use lower cut-offs for people of South Asian and some other Asian backgrounds.
- Not for children and teenagers. Under 20, BMI is interpreted on growth charts for age and sex. This calculator is for adults.
- Pregnancy. BMI is not a reliable measure during pregnancy.
Healthy weight range for your height
The "healthy weight range" row shows the weights that give a BMI between 18.5 and 24.9 at your height. It is a statistical range, not a personal target. A doctor or dietitian can take your waist size, medical history and goals into account.
Important
This calculator is for general information and is not medical advice. If you are worried about your weight, or about losing or gaining weight, speak to a doctor or another qualified health professional.
Privacy
The calculation happens in your browser. Your weight and height are not uploaded, saved or shared.
Frequently asked questions
How is BMI calculated?
BMI is weight in kilograms divided by height in meters squared. In pounds and inches it is 703 × pounds ÷ inches squared.
What is a normal BMI?
For adults, the World Health Organization lists 18.5 to 24.9 as the normal weight range.
Can I enter my weight in stones and pounds?
Yes. Choose the UK option (stones and pounds) in the units menu. You can also use kilograms or pounds.
Is BMI accurate for athletes?
Not always. BMI does not separate muscle from fat, so very muscular people can have a high BMI with low body fat.
Can I use this BMI calculator for a child?
No. It is designed for adults. For children and teenagers, BMI is read on growth charts for age and sex, so ask a doctor.
Is my weight stored or uploaded?
No. The calculation runs in your browser and nothing is saved or sent anywhere.
Sources and references
More free tools
Last updated: November 10, 2026