BMI Calculator , Metric & Imperial, Privacy-First

Body Mass Index (BMI) is a numeric value derived from a person's height and weight, calculated as weight in kilograms divided by the square of height in meters (kg/m²). The metric was introduced by Belgian statistician Adolphe Quetelet in the 1830s as the 'Quetelet Index', and renamed BMI in 1972 by physiologist Ancel Keys, who proposed it as the simplest practical proxy for population-level body fat estimation. Today the World Health Organization (WHO), the U.S. Centers for Disease Control (CDC), and most national public-health bodies use BMI categories , underweight (<18.5), normal weight (18.5–24.9), overweight (25–29.9), and obese (≥30) , as a low-cost screening tool for population health. The strength of BMI is exactly its simplicity: it requires only two measurements that nearly everyone has access to, and the calculation is something anyone can do.

This calculator handles both metric (centimeters and kilograms) and imperial (feet/inches and pounds) units, applying the appropriate formula automatically. The metric formula is BMI = weight(kg) / [height(m)]². The imperial formula is BMI = (weight(lb) × 703) / [height(in)]². Both produce the same result in the same units. The 703 multiplier in the imperial formula is the unit-conversion factor that translates pound-inch measurements into the kilogram-meter scale that BMI was originally defined on. Doing the calculation in your browser eliminates the small but real risk of arithmetic errors when entering measurements into a generic spreadsheet or doing the math by hand on a piece of paper.

BMI's most important limitation is that it does not distinguish between fat mass, muscle mass, and bone mass , only total weight. That makes BMI a poor indicator for several specific groups. Athletes with high muscle mass routinely have BMIs in the 'overweight' range despite very low body fat percentages , Olympic weightlifters and rugby players almost always classify as overweight or obese by BMI alone. Older adults experience age-related muscle loss (sarcopenia) and may have a normal or even underweight BMI while carrying excess fat mass. Children and adolescents need age- and sex-specific BMI percentile charts rather than the adult ranges. Pregnant and postpartum people are explicitly excluded from standard BMI interpretation. People who have lost a limb, have skeletal differences, or are very tall or very short can also fall outside what the index was designed for.

BMI category interpretation also varies by ethnicity. The standard WHO cutoffs were derived primarily from data on populations of European descent. Health risks at a given BMI tend to manifest at lower thresholds in many Asian populations , the WHO's separate Asian-population reference suggests action thresholds at BMI 23 (overweight) and 27.5 (obese), down from 25 and 30. Populations of African descent, conversely, tend to have higher muscle mass and bone density at the same BMI, making the standard cutoffs slightly more conservative than necessary for that group. The calculator surfaces both standard and Asian-population ranges so you can pick the more relevant context.

What BMI is genuinely useful for is tracking change in the same person over time. A 30-pound weight gain or loss for the same individual at the same height moves BMI by a meaningful amount, and the direction of that change is generally a more useful health signal than the absolute number. For someone using BMI as a personal fitness reference point, comparing this month's number to last month's , and noticing the trend , is more informative than comparing today's number to a population-level threshold. The calculator does not store your inputs, but you can record the result yourself for trend tracking outside the tool.

Modern clinical guidance increasingly pairs BMI with additional measurements that fill in BMI's blind spots. Waist circumference (or waist-to-height ratio) captures abdominal fat distribution, which independently predicts cardiovascular risk even at normal BMI. Body composition measurements (DEXA scans, bioimpedance scales, or even skinfold calipers) give a direct fat-percentage reading. Resting heart rate, blood pressure, and basic blood markers (lipid panel, fasting glucose, HbA1c) capture metabolic health more directly than weight alone. For anything beyond casual self-reference, a single BMI value is not the right basis for health decisions; combining it with these other measures gives a much more reliable picture.