BMI Calculator
Calculate Body Mass Index in metric or imperial units. WHO categories explained. Screening tool, not medical advice.
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About this tool
Body Mass Index is a quick screening number that uses your height and weight to flag whether someone might be under-, over-, or appropriately weighted for general population norms. It's a screening tool, not a diagnosis — see the limitations below.
BMI categories (adults, WHO)
- Under 18.5 — underweight
- 18.5 to 24.9 — normal range
- 25.0 to 29.9 — overweight
- 30.0 to 34.9 — obesity class I
- 35.0 to 39.9 — obesity class II
- 40.0 and above — obesity class III
Limitations
BMI is a population-level statistic and a poor individual indicator. It doesn't distinguish muscle from fat, so athletes routinely score "overweight" or "obese" by BMI while being lean. It doesn't account for bone density, body composition, fat distribution, age, sex, or ethnicity. For children and teens, BMI percentiles for age and sex are the appropriate measure — not the adult categories.
Use this as a starting conversation with your doctor, not as a self-diagnosis.
Frequently asked questions
- Why is my BMI in the "overweight" range when I'm clearly not?
- BMI is a height/weight ratio that doesn't see body composition. Muscular people score high because muscle is denser than fat. If you carry significant muscle mass, BMI undercounts your actual health markers. Waist circumference, body fat percentage, and metabolic markers are better individual indicators.
- Does this work for kids?
- The formula is the same but the categories aren't. For people under 20, BMI is interpreted using growth charts that compare against same-age, same-sex percentiles. The tool will flag this when an age below 20 is entered.
- How accurate are the numbers?
- The calculation is exact. Whether BMI is meaningful for you is the question — see the limitations above.
- Is this a substitute for medical advice?
- No. It's a screening estimate. Your doctor has the context (medications, family history, body composition, blood work) to interpret it usefully.
Last updated: May 17, 2026