BMI Calculator

Body Mass Index is a simple numerical metric calculated from height and weight: BMI = weight(kg) / height(m)². In imperial units: BMI = 703 × weight(lb) / height(in)². The World Health Organization defines standard adult BMI categories: underweight (< 18.5), normal weight (18.5–24.9), overweight (25–29.9), and obese (≥ 30), with obesity further subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (≥ 40).

BMI was developed in the 1830s by Belgian statistician Adolphe Quetelet as a population-level statistical tool, not as an individual diagnostic. Its widespread clinical adoption came in the 1970s-1980s. While BMI correlates with body fat percentage at population scale and with cardiovascular disease risk in large studies, it has significant limitations as an individual health assessment: it doesn't distinguish between muscle and fat mass (highly muscular individuals often register as 'overweight'), doesn't account for fat distribution (visceral vs. subcutaneous), and has different accuracy across ethnic populations.

For children and adolescents, BMI is interpreted differently using age- and sex-specific percentile charts rather than the adult absolute category thresholds.

BMI is one of several anthropometric measures used to assess body composition and cardiometabolic risk. Waist circumference is arguably more clinically relevant than BMI because abdominal (visceral) fat is more metabolically active and strongly associated with insulin resistance, type 2 diabetes, and cardiovascular disease. WHO thresholds: waist > 94 cm (men) / 80 cm (women) indicates increased risk; > 102 cm / 88 cm indicates substantially increased risk. Waist-to-height ratio (WHtR) < 0.5 is a simple rule of thumb for reduced cardiometabolic risk. Body fat percentage measured via DEXA scan, hydrostatic weighing, or bioelectrical impedance provides the most direct body composition assessment but requires equipment beyond this calculator.