BMI Calculator Online - Free Body Mass Index
Free online BMI calculator with metric and US units. Get WHO adult classification, healthy weight range, BMI Prime, and Ponderal Index - privately in your browser.
Ages 2–120
Optional context only
Values update as you type. Classification uses WHO adult BMI ranges. Sex is optional and does not change the formula.
Your BMI
22.9kg/m²
Normal
- Healthy BMI range: 18.5–25 kg/m²
- Healthy weight for this height: 56.7–76.6 kg
- BMI Prime: 0.91
- Ponderal Index: 13.1 kg/m³
Educational use only - not medical advice or a diagnosis. BMI does not measure body fat directly and can misclassify athletes, older adults, pregnancy, and some ethnic groups. Talk to a clinician about your health.
Free online BMI calculator
Body Mass Index is a quick height–weight screening number used worldwide. Enter your details above to get an adult WHO classification, a healthy weight band for your height, BMI Prime, and Ponderal Index. Nothing is uploaded - the math runs in your browser.
Metric and US units
Switch between cm/kg and ft/in/lb. Results update as you type.
Full WHO adult scale
See thinness through Obese Class III, plus a visual category gauge.
Extra metrics
Healthy weight for your height, BMI Prime, and Ponderal Index - privately in the browser.
How to use this tool
- Choose Metric or US units.
- Enter age (and optionally sex for context).
- Enter height and weight - results update automatically.
- Read the classification, gauge, and healthy weight range.
- If you are under 20, note that pediatric percentiles are not computed here.
What BMI measures
BMI estimates body mass relative to height. Public-health organizations use it as a general indicator of underweight, normal weight, overweight, or obesity for adults. It does not measure fat percentage, fitness, or disease risk for an individual.
WHO BMI table for adults
Recommended adult categories (kg/m²), commonly applied for ages 20 and older:
| Classification | BMI (kg/m²) |
|---|---|
| Severe Thinness | < 16 |
| Moderate Thinness | 16 – 17 |
| Mild Thinness | 17 – 18.5 |
| Normal | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obese Class I | 30 – 35 |
| Obese Class II | 35 – 40 |
| Obese Class III | ≥ 40 |
BMI formula
Metric (SI)
BMI = weight (kg) / [height (m)]²
Example: 72.57 kg at 1.778 m → BMI ≈ 23.0
US customary
BMI = 703 × weight (lb) / [height (in)]²
Example: 160 lb at 70 in (5′10″) → BMI ≈ 23.0
BMI Prime and Ponderal Index
BMI Prime is BMI ÷ 25 (the usual upper end of the adult normal range). Near 1 means near that reference. Ponderal Index uses height cubed (kg/m³) and can complement BMI for very tall or short adults. Both are educational extras, not diagnoses.
Children and teens (ages 2–20)
For children and adolescents, clinicians typically use CDC BMI-for-age percentile charts (underweight, healthy weight, at risk of overweight, overweight) rather than adult WHO cutoffs. This calculator does not compute those percentiles. If age is under 20, we show a notice and still display the adult WHO category for reference only.
Limitations of BMI
- BMI estimates excess weight, not excess fat.
- High muscle mass can raise BMI without high body fat.
- Age, sex, and ethnicity influence body composition at the same BMI.
- Pregnancy and some medical conditions change how weight should be interpreted.
- Use BMI alongside clinical judgment and other measures - not alone.
General health context
Population research associates very high or very low BMI with higher rates of certain conditions (for example metabolic, cardiovascular, bone, or nutritional concerns). That does not mean a single BMI number diagnoses disease for you. Discuss results and next steps with a qualified clinician.