Calculate your Body Mass Index (BMI) using imperial or metric measurements.
Body Mass Index (BMI) was developed by Belgian statistician Adolphe Quetelet in the 1830s as a measure of average body weight relative to height for population-level statistical analysis. It was not designed to be a clinical diagnostic tool for individuals. BMI became widely used in health screening because it is simple to calculate and correlates reasonably well with body fat percentage at the population level, even though it can be significantly misleading for specific individuals.
The formula — weight in kilograms divided by the square of height in meters — produces a dimensionless number that classifies individuals into underweight (under 18.5), normal weight (18.5–24.9), overweight (25–29.9), and obese (30+) categories. These thresholds were established based on statistical associations between BMI and health outcomes in large population studies.
The most widely discussed limitation of BMI is its inability to distinguish between muscle mass and fat mass. A competitive bodybuilder might weigh 220 pounds at 5'10" (a BMI of 31.6, classified "obese") while having under 10% body fat — an extremely lean physique by any meaningful measure. Conversely, a sedentary person with the same BMI might have normal weight with high body fat percentage (sometimes called "normal weight obesity" or "skinny fat").
Research has shown that where fat is stored matters more than total fat percentage for health risk assessment. Visceral fat (stored around abdominal organs) is far more metabolically harmful than subcutaneous fat (stored under the skin). BMI provides no information about fat distribution. Waist circumference and waist-to-hip ratio are better predictors of metabolic disease risk than BMI for many individuals.
BMI classifications were developed primarily from studies of European populations and may not apply uniformly across ethnic groups. Research suggests that Asian populations may experience higher metabolic risks at lower BMI values — many health organizations now recommend lower BMI thresholds for Asian adults: overweight at 23, obese at 27.5. Black Americans, particularly women, may have lower risk at higher BMIs compared to white Americans at the same BMI.
Age also complicates BMI interpretation. The muscle loss that naturally occurs with aging (sarcopenia) means an older adult with a "healthy" BMI may have relatively high body fat and low muscle mass. For children and teenagers, BMI is interpreted differently using sex- and age-specific growth charts rather than fixed adult thresholds.
Healthcare providers increasingly use BMI as one data point among several rather than a standalone diagnostic. Complementary measures include waist circumference (elevated risk: over 35 inches for women, 40 inches for men), waist-to-hip ratio, DEXA (dual-energy X-ray absorptiometry) body composition scans for accurate fat and muscle measurement, and bloodwork including fasting glucose, cholesterol panel, and inflammatory markers.
The simplest additional measurement is waist circumference, which requires only a measuring tape and strongly predicts metabolic disease risk independent of BMI. A person with "normal" BMI but large waist circumference may have higher cardiovascular disease risk than someone with higher BMI and smaller waist.