BMI Calculator
Body Mass Index from height and weight, with WHO category classification
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What this calculator computes
Body Mass Index (BMI) is the most common screening tool for adult body-weight classification, computed as weight in kilograms divided by height in metres squared (or weight in pounds divided by height in inches squared, multiplied by 703 for the imperial form). The calculator accepts either metric or imperial inputs and returns the BMI value along with the World Health Organization category: underweight (under 18.5), healthy weight (18.5 to 24.9), overweight (25 to 29.9), obesity class I (30 to 34.9), class II (35 to 39.9), and class III (40 and above). BMI was developed by Belgian statistician Adolphe Quetelet in the 1830s as a population-level metric and was popularised as an individual-level screening tool by the US National Institutes of Health in the 1990s. The metric is widely used in primary-care intake, public-health surveillance, and clinical-research stratification because it is cheap to compute and correlates moderately well with body-fat percentage at the population level. Its limitations are well-documented: it overestimates body fat in muscular athletes (a 6-foot, 220-pound bodybuilder shows BMI 29.8 in the "overweight" range despite low body fat), underestimates risk in older adults with sarcopenia, performs poorly in pregnancy, and is not validated for children where age-and-sex-adjusted percentiles are used instead.
Calculator
The formula
Formula
BMI = weight (kg) / height (m)² or BMI = weight (lb) / height (in)² × 703
Worked example
When to use this calculator
Use BMI as a screening tool in primary-care intake, fitness-app onboarding, and population-health surveillance, where the cheap-and-fast nature of the metric outweighs its individual-level inaccuracies. The calculator is most useful for sedentary or moderately active adults aged roughly 20 to 65 of average build; it is least useful for muscular athletes, elderly adults with reduced lean mass, pregnant women, and children. For athletes, supplement BMI with body-fat-percentage measurement (DEXA scan, bioelectrical impedance, or skinfold calipers) and waist-to-hip ratio. For children, use age-and-sex-adjusted BMI percentiles published by the CDC and WHO rather than adult cutoffs. The calculator does not diagnose obesity or any health condition; clinical interpretation requires a physician who can assess individual risk factors alongside the BMI value.
Common input mistakes
- Applying adult BMI cutoffs to children. Children's BMI changes rapidly with age and differs by sex, so the adult thresholds (25 overweight, 30 obese) do not apply. Pediatric BMI should be plotted on the CDC growth chart against age-and-sex-adjusted percentiles, with the 85th percentile typically the overweight threshold and the 95th percentile the obese threshold.
- Treating BMI as a diagnosis rather than a screening tool. A muscular athlete with BMI 28 is not "overweight" in any clinically meaningful sense; an elderly person with BMI 22 may have lost dangerous amounts of muscle mass while remaining "healthy" by BMI alone. Always interpret BMI alongside body composition, waist-to-hip ratio, and the patient's overall health context.
Frequently asked questions
What are the WHO BMI categories?
The World Health Organization defines six adult BMI categories: underweight (below 18.5), healthy weight (18.5 to 24.9), overweight (25.0 to 29.9), obesity class I (30.0 to 34.9), obesity class II (35.0 to 39.9), and obesity class III or "severe obesity" (40 and above). The thresholds are derived from large-population studies correlating BMI ranges with mortality and metabolic disease risk. Some Asian-population studies suggest lower thresholds (overweight at 23, obesity at 27.5) because cardiovascular risk increases at lower BMI in Asian populations than in European populations.
Is BMI accurate for athletes?
BMI overestimates body fat in muscular athletes because it counts lean muscle mass the same as fat mass. A 6-foot, 220-pound bodybuilder with 8% body fat shows BMI 29.8 in the "overweight" category, while a sedentary 6-foot, 220-pound person with 30% body fat shows the same BMI but very different metabolic risk. Athletes and habitual lifters should use body-fat-percentage measurement (DEXA, bioelectrical impedance, or calipers) alongside BMI for a meaningful body-composition assessment.
How does BMI work for children?
Children's BMI is calculated the same way as adult BMI but interpreted against age-and-sex-adjusted percentile charts published by the CDC and WHO. A child whose BMI is at the 85th percentile for their age and sex is considered overweight; the 95th percentile is the obese threshold. Adult cutoffs do not apply because children's body composition changes rapidly during growth, and what is "high BMI" at age 6 may be "normal BMI" at age 14 due to developmental factors.
What is a healthy BMI?
For most adults aged 20 to 65 of average build, the WHO-defined healthy BMI range is 18.5 to 24.9. The lower bound prevents underweight-associated risks (osteoporosis, immune compromise, sarcopenia in older adults), and the upper bound prevents the metabolic and cardiovascular risks associated with overweight and obesity. Within the healthy range, individual optima vary based on body composition, muscle mass, and ethnic background; BMI 22–23 is often cited as the sweet spot for cardiovascular outcomes in epidemiological data.
Are there better alternatives to BMI?
Body-fat percentage (measured by DEXA scan, hydrostatic weighing, bioelectrical impedance, or skinfold calipers) is more accurate but requires specialized equipment. Waist-to-hip ratio and waist-to-height ratio capture central obesity more directly and correlate better with cardiovascular risk than BMI alone. The "no single number" approach combines BMI with one or more of these metrics for a complete picture; a person with BMI 27, body fat 18%, and waist-to-height ratio 0.45 has very different risk than another person with the same BMI but body fat 35% and waist-to-height ratio 0.55.