BMI Calculator

BMI = weight (kg) / height (m)². The Quetelet index, invented in the 1830s as a population-statistics tool, repurposed in the 1970s as a screening metric. The WHO categories for adults of European descent: under 18.5 underweight, 18.5–24.9 normal, 25.0–29.9 overweight, 30.0+ obese (with class I 30–34.9, II 35–39.9, III 40+).

For East Asian and South Asian populations, the WHO recommends lower thresholds — overweight starts at 23, obese at 27.5 — because cardiometabolic risk increases at lower BMI in these groups. Many clinical guidelines globally have not adopted the ethnicity-adjusted thresholds.

• Muscular individuals — bodybuilders and athletes routinely have BMI 28-32 with 8% body fat. Classified obese by BMI, in excellent metabolic health by every other marker.
• Older adults — BMI does not capture sarcopenic obesity (low muscle, high fat). An elderly person with "normal" BMI can be severely metabolically unhealthy.
• Children — pediatric BMI uses percentile-for-age charts, not the adult cutoffs. Applying adult BMI categories to children is wrong.
• Pregnancy — BMI categories do not apply during pregnancy. Use pre-pregnancy BMI for gestational weight gain recommendations.
• Very short / very tall — the height-squared denominator is empirically a poor fit at the extremes. Some researchers prefer BMI = weight / height^2.5 (Trefethen index) which adjusts more sensibly for tall people.

• Waist-to-height ratio — keep your waist circumference under half your height. Better predictor of cardiometabolic risk than BMI in most studies. Easy to measure yourself.
• Waist circumference alone — over 94 cm (men) / 80 cm (women) increased risk; over 102 cm (men) / 88 cm (women) high risk. WHO thresholds; specific guidelines vary slightly by region.
• Body fat percentage — DXA scan is gold standard. Skinfold calipers and bioelectrical impedance are cheap but error-prone (±3-5%). Healthy ranges vary by age, sex, and athletic status.
• Cardiometabolic markers — fasting glucose, HDL/LDL/triglycerides, blood pressure, resting heart rate. These predict outcomes; BMI is a proxy for these.
• Functional fitness — grip strength, VO2 max, time to climb four flights of stairs. Mortality correlates more strongly with these than with body composition.

• You want a single screening number to track over time — BMI changes correlate with weight changes at constant height, useful for monitoring trends.
• A health questionnaire asks for BMI and you want to compute it from height/weight you know.
• You are curious where you fall on the WHO categories and want to know how concerned (or not) to be.
• You are coaching someone and want a starting reference number to motivate body-composition tracking (paired with at least one of the better metrics).

• It will not diagnose anything. BMI is a screening number, not a diagnostic test. "Obese by BMI" does not mean "unhealthy"; "normal by BMI" does not mean "healthy".
• It will not give medical advice. If your BMI falls outside the normal range and you are concerned, see a clinician — they will pair BMI with bloodwork, history, and context.
• It will not adjust for ethnicity automatically — you select the WHO general or Asian thresholds. There is no universal correct answer; choose the one your clinical guidelines reference.