BMI calculator
Body mass index (BMI) is your weight in kilograms divided by your height in meters squared. The calculator works it out and gives the WHO category for adults.
The categories follow the WHO classification: under 18.5 is underweight, 18.5–24.9 a healthy weight, 25–29.9 overweight, and 30 or above obesity. WHO further divides obesity into three classes, starting at 30, 35 and 40.
BMI knows only your height and weight. It cannot tell muscle from fat or show where the fat is stored, so for an individual it is a guide, not a diagnosis.
How it works
- Enter your height in centimeters and your weight in kilograms.
- Read the number and the category: they apply to adults from 18.
- If your BMI is outside the healthy range or raises questions, talk it over with a doctor, who can take into account what the formula cannot see.
When BMI gets it wrong
The UK’s NICE guideline on overweight and obesity advises interpreting BMI with caution in adults with high muscle mass, and it is not used to judge weight during pregnancy. For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, NICE uses lower thresholds: overweight from 23 and obesity from 27.5, because health risks rise at a lower BMI.
Adult cut-offs do not apply to children and teenagers: their BMI is compared with the WHO reference for age and sex. That is why the calculator gives no category under 18.
| BMI | WHO category |
|---|---|
| under 18.5 | underweight |
| 18.5–24.9 | healthy weight |
| 25–29.9 | overweight |
| 30 and above | obesity |
Frequently asked questions
- How do I calculate BMI by hand?
- Divide your weight in kilograms by your height in meters squared. For example, 70 kg at 1.70 m: 1.70 × 1.70 = 2.89, and 70 ÷ 2.89 ≈ 24.2.
- Are the BMI categories the same for everyone?
- For adults WHO uses one scale. A WHO expert consultation in 2004 proposed additional public health cut-offs of 23 and 27.5 for Asian populations, whose risks rise at a lower BMI, and NICE applies the same lower thresholds to several ethnic groups. The main scale itself was kept.
- Why can an athlete come out as overweight?
- Muscle is denser than fat, and the formula sees only total weight. With a lot of muscle, BMI can go above 25 at a low body fat percentage. In such cases waist measurements and body composition tell you more.
Further reading
- Body mass index: how to calculate it and where it goes wrong
- The scale is steady but your waist is shrinking
- TDEE calculator: total daily energy expenditure
More calculators: Ideal weight calculator, Body fat calculator, Waist-to-height and waist-to-hip ratio calculator, Water intake calculator.
This calculator gives an estimate for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.