BMI Calculator
How This Calculator Works
The math is simple: weight in kilograms divided by height in meters, squared.
BMI = weight(kg) / height(m)²
WHO categories (adults 20+):
< 18.5 Underweight
18.5 – 24.9 Normal weight
25.0 – 29.9 Overweight
30.0 – 34.9 Obesity class I
35.0 – 39.9 Obesity class II
≥ 40 Obesity class IIIOrigin: Quetelet A. Sur l'homme et le développement de ses facultés. Brussels, 1832 (the original derivation). Modern categories: WHO Technical Report Series 894, "Obesity: Preventing and Managing the Global Epidemic," 2000.
What BMI Tells You — And What It Doesn't
BMI is a population-level screening tool. It doesn't distinguish muscle from fat, doesn't account for frame size, and doesn't know where your fat sits. A muscular athlete can score "obese." A sedentary person with high body fat percentage can score "normal." Treat BMI as one data point among many — not a diagnosis.
Limitations
BMI was derived from European population averages in the 1800s. It tends to underestimate body fat in some Asian populations (which is why some countries use lower cutoffs) and overestimate it in muscular athletes. It doesn't apply to children, pregnant people, or elderly people with significant muscle loss without adjustment. For body composition, a body fat percentage measurement is more useful; for health risk, waist circumference and waist-to-hip ratio often predict cardiovascular risk better than BMI alone.
Frequently Asked Questions
What is BMI?
BMI — Body Mass Index — is a single number derived from your weight and height. The formula is weight in kilograms divided by height in meters squared. The math is simple, and the number itself is meant to be a quick, population-level screening tool — not a diagnosis.
What are the BMI categories?
The WHO categories for adults are: under 18.5 = underweight; 18.5–24.9 = normal weight; 25.0–29.9 = overweight; 30.0–34.9 = obesity class I; 35.0–39.9 = obesity class II; 40 and above = obesity class III.
Is BMI accurate?
It's a population-level tool, not a personal one. BMI doesn't tell muscle from fat, doesn't account for frame size, doesn't know where your fat sits, and was originally derived from European populations. A muscular athlete can score 'obese' on BMI; a sedentary person with high body fat percentage can score 'normal.' Useful for screening big groups, lousy as a personal verdict.
Why do bodybuilders get classified as obese?
Because BMI only sees weight and height. Muscle is denser than fat — about 18% denser by volume — so a muscular person at the same height weighs more than a sedentary person of similar build. The math labels both 'overweight' or 'obese' without any clue why. This is the most famous limitation of BMI.
Should I use BMI to set weight goals?
Use it as a rough sanity check, not a target. Hitting 'normal BMI' isn't the same as being healthy — body fat percentage, fitness level, and metabolic health matter more. If your BMI is well outside the normal range and you don't carry obvious extra muscle, it's a reasonable signal to talk to a doctor.
Does BMI work for kids?
No, not directly. Children and teens use BMI-for-age percentiles instead, because their height and body composition are still changing. The simple formula and category cutoffs in this calculator are for adults aged 20+. If you're calculating for a child, look up CDC BMI percentile charts.
What's a better measure than BMI?
For body composition, body fat percentage (DEXA scan, BIA, or US Navy method) is more accurate. For health risk, waist circumference and waist-to-hip ratio predict cardiovascular risk better than BMI alone. For fitness, lab-tested VO2 max or resting heart rate. BMI is fine as one data point — just not the only one.
Where did BMI come from?
Belgian astronomer and statistician Adolphe Quetelet derived it in the 1830s as a way to describe the 'average man' in population statistics. He never intended it as a measure of individual health — that came much later when actuaries and the WHO adopted it. The cutoff values used today come from the WHO Technical Report Series 894 (2000).