BMI calculator

What BMI actually measures

Body mass index is a single number derived from your height and weight. It's used by the World Health Organization, public health bodies, and life-insurance underwriters as a quick screening metric for population-level health risk. It's a screen, not a diagnosis — useful precisely because it's easy and fast.

What BMI doesn't know

BMI doesn't know about muscle mass, bone density, frame size, or where your body stores fat. That's why strength athletes often land in the "overweight" band while being visibly lean, and why a sedentary person at a "healthy" weight can still carry a metabolically risky amount of visceral fat. If you want a fuller picture, pair BMI with the body-fat calculator and a tape-measured waist-to-height ratio.

The formula

Metric: kg ÷ (m × m)
Imperial: 703 × lb ÷ (in × in)

The 703 multiplier in the imperial version is just a unit conversion so the result lands on the same scale as the metric one — that way both versions share the same bands.

BMI bands (adults, WHO)

• Under 18.5 — underweight
• 18.5–24.9 — healthy
• 25–29.9 — overweight
• 30–34.9 — obese class I
• 35–39.9 — obese class II
• 40 and above — severely obese

These bands are calibrated for adults aged 20+. For children and adolescents, BMI is read off age-specific percentile charts instead. Pregnant women, the elderly, and competitive athletes should treat BMI with extra skepticism.

How to use the result

If you land outside the healthy band, the next question isn't "how do I get back in" — it's "does this match my actual measurements?". Cross-check with a tape-measured waist (under 94 cm for men, under 80 cm for women is generally low-risk) and your body fat percentage. If those line up with the BMI band, the band is probably telling you something real.