BMI calculator

BMI: a useful population metric with significant individual limitations

Body mass index (BMI) is calculated as weight in kilograms divided by height in meters squared. For imperial measurements, the equivalent is: 703 × weight_lbs / height_in². The result is a dimensionless number that WHO uses to classify weight categories at the population level.

The Quetelet origin: statistics, not medicine

BMI was invented by Adolphe Quetelet, a Belgian mathematician, in the 1830s. Quetelet called it the “Quetelet Index” and designed it as a tool for describing the statistical distribution of body size in populations — not for diagnosing individual health. He used it to model the “average man” in census data.

The index was adopted for clinical and public health use in the 1970s–1980s when epidemiologists needed a cheap, reproducible measurement to study obesity trends in large populations. It has since been used far beyond its original scope, applied to individual health judgments it was never designed to make.

Why BMI is still used

Despite limitations, BMI survives because:

• It's cheap and reproducible — requires only a scale and a measuring tape, takes 30 seconds, and produces consistent results across observers. DEXA scans cost $50–200 and require specialized equipment.
• It correlates with population-level outcomes — in large epidemiological studies, BMI categories do track with increased risk of type 2 diabetes, hypertension, coronary artery disease, and certain cancers. The correlation is real, even if causation is complex.
• It captures the average person reasonably well — among sedentary individuals with average muscle mass, BMI is a reasonable proxy for adiposity. The problem is when it's applied to athletes, very muscular individuals, elderly people with low muscle mass, or different ethnic groups.

Well-documented limitations

• Muscle vs fat — BMI cannot differentiate. A 5'10”, 200-lb competitive bodybuilder and a 5'10”, 200-lb sedentary person have identical BMIs but radically different cardiometabolic profiles.
• Ethnic variation — Asian populations have higher metabolic risk at lower BMI values. WHO has proposed lower cutoffs for South, East, and Southeast Asian populations (overweight at ≥23, obesity at ≥27.5).
• Age effects — in older adults (65+), BMI 25–27 (technically “overweight”) is associated with lower all-cause mortality than normal BMI — the “obesity paradox” (Flegal et al. 2013, JAMA). Some of this represents preserved lean mass.
• Fat distribution — BMI tells you nothing about where fat is stored. Visceral fat (intra-abdominal) is metabolically active and pro-inflammatory; subcutaneous fat (under the skin) is less so. Two people with identical BMIs can have very different visceral fat loads.

Better metrics for individual assessment

Waist-to-height ratio (WHtR) is the most evidence-backed simple alternative. The target: waist circumference (inches) divided by height (inches) less than 0.5. A 5'10” person (70 inches) should have a waist under 35 inches. This ratio correlates better with visceral fat, insulin resistance, and cardiovascular risk than BMI across multiple ethnicities and age groups.

Waist circumference alone: risk thresholds are 40 inches for men, 35 inches for women, regardless of height or BMI.

Body fat percentage: assessed by DEXA, hydrostatic weighing, or the U.S. Navy circumference method (free calculator on this site). Athletic body fat targets: 10–15% for men, 18–24% for women.