BMI Calculator

BMI, or Body Mass Index, is a simple screening number calculated from weight and height: weight in kilograms divided by height in meters squared. It is widely used because it is quick, cheap, and easy to compare across large groups. This calculator helps turn height and weight into a BMI value and then compares it with selected reference standards, such as WHO or regional Asian thresholds. BMI can support a first health check, fitness log, or weight-range estimate, but it does not measure body fat directly. Muscular athletes, children, older adults, pregnant people, and people with unusual body composition may be misclassified, so medical decisions need professional context.

Body Mass Index is the Quetelet Index, proposed in 1832 by Belgian mathematician Adolphe Quetelet and renamed BMI by Ancel Keys in 1972. The formula is BMI = mass(kg) / height(m)^2, a ratio with units of kg/m^2. Squaring the height was an empirical choice to make the index roughly independent of stature in adult populations of European ancestry; it is known to over-classify tall people and under-classify short people because human mass actually scales closer to height^2.7 in some growth studies. Imperial inputs are converted with 1 lb = 0.45359237 kg and 1 in = 0.0254 m (NIST exact definitions) before the formula is applied. The classification bands depend on the regional standard. The WHO 1995 international cutoffs are <18.5 underweight, 18.5-24.9 normal, 25.0-29.9 overweight, and ≥30 obese (split into class I 30-34.9, class II 35-39.9, class III ≥40). The Asia-Pacific overweight threshold is lower because the same BMI corresponds to a higher body-fat percentage in many East and South Asian populations (WHO/IASO/IOTF 2000; WHO expert consultation 2004 proposed additional public-health action points at 23 and 27.5). China adopts WS/T 428-2013 (and earlier the 2003 Working Group on Obesity in China guideline) with bands <18.5, 18.5-23.9, 24-27.9, ≥28, and Japan's JASSO uses ≥25 as obese class I. The healthy-weight range is the inverse of the formula: lower = BMI_low * height^2, upper = BMI_high * height^2, where the bounds come from the selected standard's normal band. The ideal-weight figure typically uses BMI = 22, the midpoint that minimizes all-cause mortality in several Asian cohort studies. BMI's well-documented blind spots include muscular athletes (high lean mass), sarcopenic elderly (low lean mass with high fat), pregnancy, pediatric growth, and ascites or edema; supplementary measures such as waist circumference (China cutoff: male ≥90 cm, female ≥85 cm), waist-to-hip ratio, and bio-impedance body-fat percentage are recommended for clinical decisions.

• Quetelet formula: BMI = weight(kg) / height(m)^2; unit kg/m^2; defined for adults aged 20+ only, pediatric assessment uses CDC/WHO percentile growth charts instead
• Imperial conversion uses NIST exact factors: 1 lb = 0.45359237 kg and 1 in = 0.0254 m, so a BMI from lb/in inputs equals weight_lb / height_in^2 * 703
• WHO 1995 international bands: <18.5 / 18.5-24.9 / 25.0-29.9 / ≥30 (class I 30-34.9, II 35-39.9, III ≥40 morbid obesity)
• Asia-Pacific bands (WHO 2004, China WS/T 428-2013 and the 2003 Working Group on Obesity in China guideline): <18.5 / 18.5-23.9 / 24-27.9 / ≥28, reflecting higher metabolic risk at lower BMI in East and South Asian populations; Japan JASSO uses ≥25 as obese class I
• Healthy weight range derived by inversion: weight_min = 18.5 * height^2, weight_max = upper_normal * height^2; ideal weight commonly set at BMI = 22 (lowest mortality in Asian cohorts)
• Known biases: over-classifies muscular athletes (lean mass reads as fat), under-classifies sarcopenic elderly, and is invalid during pregnancy and for children under 20
• Companion metrics: waist circumference (China ≥90 cm male / ≥85 cm female), waist-to-hip ratio, and body-fat percentage via Deurenberg estimate BF% = 1.20*BMI + 0.23*age - 10.8*sex - 5.4 (sex: male=1, female=0)