BMI Calculator
Guide & FAQGuide: BMI Calculator
What BMI measures
Body mass index compares your weight with your height. It is used worldwide to screen adults for possible weight-related health risks. It is a screening tool, not a diagnosis, and it cannot tell fat from muscle.
The formulas
- Metric: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = 703 × weight (lb) ÷ height (in)²
- Healthy weight range: the weights that give a BMI between 18.5 and 25 at your height, i.e. 18.5 × h² to 25 × h²
Example: 70 kg at 175 cm is 70 ÷ 1.75² = 22.9, which is in the healthy range (about 56.7–76.6 kg for that height).
WHO adult categories
- Below 18.5: underweight (thinness)
- 18.5 to 24.9: healthy weight
- 25 to 29.9: overweight
- 30 and above: obesity, in three classes
Limits you should know
BMI ignores muscle mass, body frame, age, sex and where fat is stored. Some ethnic groups face higher health risk at lower BMI values, and athletes often read high. Adult ranges apply from age 18; for children use growth charts. A result outside the healthy range is a reason to ask a professional, not a verdict. Nothing you enter leaves your browser.
FAQ
What is BMI?
Body mass index is weight divided by height squared (kg/m²). It is a quick way to screen for weight categories in adults.
What are the BMI categories?
WHO adult categories: under 18.5 underweight, 18.5–24.9 healthy weight, 25–29.9 overweight, 30 and above obesity (class I 30–34.9, II 35–39.9, III 40+).
Is BMI accurate?
Only as a rough screen. It does not measure body fat, muscle, bone density, age or sex, so muscular people can read as overweight and older adults can hide low muscle mass.
Does it work for children?
No. Children and teens are assessed with BMI-for-age percentile charts. This calculator is for adults aged 18 and over.
What should I do with the result?
Use it as a starting point for a conversation with a doctor or dietitian, together with waist size, fitness, diet and health history.