BMI Calculator
Find your Body Mass Index and WHO weight category.
BMI Calculator
Calculate your Body Mass Index and see which WHO weight category you fall into.
BMI is a screening tool, not a diagnostic measure of body fat or health. It cannot distinguish muscle from fat — a muscular athlete may be classified 'overweight.' For a true body composition breakdown, a DEXA scan is available at most sports medicine clinics.
About this calculator
BMI (Body Mass Index) is a ratio of weight to height squared, used by the WHO to screen for underweight, healthy weight, overweight, and obesity. While it does not directly measure body fat, it is a fast, cost-free proxy that correlates with metabolic health risk at the population level. The four standard categories are: Underweight (<18.5), Normal (18.5–24.9), Overweight (25–29.9), and Obese (≥30). BMI is most reliable for sedentary adults aged 18–65 and less accurate for athletes, older adults, or people with high muscle mass, who may score "overweight" despite low body fat.
How to interpret your results
A BMI in the 18.5–24.9 range suggests healthy weight-for-height proportion and is associated with the lowest risk of metabolic disease. If your BMI is 25–29.9 (overweight), the primary action is to assess waist circumference — if your waist is below 94 cm (men) or 80 cm (women), your health risk may still be low despite the BMI classification. BMI of 30 or above warrants a more complete assessment including blood pressure, fasting glucose, and lipid panel. At the underweight end (below 18.5), the concerns are nutritional deficiency, low bone density, and reduced immune function. Athletes who score in the overweight range despite visible leanness should use body fat percentage as their primary metric — their elevated weight is muscle, not fat.
The science and formula
The BMI formula — weight (kg) ÷ height² (m²) — was developed by the Belgian mathematician Adolphe Quetelet in 1832 as a statistical tool to describe population weight distributions, not individual health. Ancel Keys coined the term "Body Mass Index" in 1972 and validated its use as a population-level screening tool. The WHO adopted current cut-offs in 1995. The formula scales weight by the square of height because body volume (and thus mass) scales roughly as the cube of a linear dimension, but height-squared provides a good empirical approximation for population averages. Imperial BMI = (weight in lbs × 703) ÷ height in inches².
Important limitations
BMI has well-documented limitations. It cannot distinguish fat mass from muscle mass — a muscular athlete and a sedentary person of the same height and weight will have identical BMIs but vastly different health profiles. It does not account for fat distribution; abdominal fat carries higher risk than the same amount of fat distributed peripherally. Asian populations have higher metabolic risk at lower BMI values — many Asian health agencies recommend lower cut-offs (23 for overweight, 27.5 for obese). BMI is also less reliable for children, pregnant women, older adults with sarcopenia, and people under 5 feet tall.
Frequently Asked Questions
Is BMI an accurate measure of body fat?
BMI is a screening tool, not a direct fat measurement. It overestimates fatness in muscular people and can underestimate it in older adults who have lost muscle. For a more precise picture, combine BMI with waist circumference or body fat percentage.
What is a healthy BMI range for adults?
The WHO defines 18.5–24.9 as the healthy range for adults. BMI 25–29.9 is overweight, and 30 or above is classified as obese. Underweight is below 18.5. Thresholds differ slightly for Asian populations.
Does BMI apply to children and teenagers?
Children use age- and sex-specific BMI percentile charts, not adult cut-offs. A BMI at or above the 85th percentile is considered overweight for children; at or above the 95th percentile is obese.
Can two people with the same BMI have very different health risks?
Yes, significantly. Two people at BMI 27 — one a muscular athlete and one a sedentary person with central obesity — face completely different health risk profiles. This is why waist circumference and body fat percentage complement BMI for a complete assessment.
Why do Asian countries use different BMI cut-offs?
Research shows that people of Asian descent develop metabolic complications (type 2 diabetes, hypertension) at lower BMI values than Western populations. Many Asian health agencies use 23.0 as the overweight threshold and 27.5 as obese, rather than the WHO standard of 25 and 30.
Does BMI change as you age?
BMI changes with body composition even at constant weight, because muscle mass declines with age (sarcopenia) while fat mass tends to increase. An older adult can have a "healthy" BMI while carrying excess body fat and too little muscle. For people over 65, body weight slightly above the normal BMI range may actually be protective.
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