BMI Calculator

Body Mass Index (BMI), originally formulated as the Quetelet Index in the 19th century by Belgian mathematician Adolphe Quetelet, is an epidemiological screening metric adopted worldwide by the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC).

BMI evaluates the relationship between body weight and stature by dividing mass in kilograms by height in meters squared (kg/m²). For imperial measurements, the formula scales by a factor of 703: 703 × weight (lbs) ÷ height (inches)². While simple, BMI provides a non-invasive, universally replicable benchmark for population-level health assessments and risk stratification.

The World Health Organization categorizes adult BMI into four primary classifications: Underweight (< 18.5), Normal / Healthy Weight (18.5 to 24.9), Overweight (25.0 to 29.9), and Obesity (Class I: 30.0 to 34.9; Class II: 35.0 to 39.9; Class III: ≥ 40.0).

Clinical studies consistently link elevated BMI values above 25 with increased risks of cardiovascular disease, type 2 diabetes mellitus, hypertension, and obstructive sleep apnea. Conversely, a BMI below 18.5 is correlated with risks of malnutrition, bone density depletion (osteoporosis), and impaired immune function.

Although BMI is an effective statistical screening tool, it does not directly measure body composition, subcutaneous adipose tissue, or visceral fat distribution. Highly conditioned athletes and bodybuilders with substantial skeletal muscle mass frequently register as "overweight" or "obese" despite possessing very low body fat percentages.

Similarly, in elderly populations, sarcopenia (age-related muscle loss) can mask increased relative adiposity within a "normal" BMI reading. For a comprehensive clinical evaluation, healthcare practitioners pair BMI with waist-to-hip ratio (WHR), bioelectrical impedance, or DEXA scans.