Body Mass Index is weight divided by height squared. It was devised in the 1830s as a way of describing populations, and it is still used because it is quick and needs no equipment. It is a screening indicator, not a diagnosis.
How to use the BMI Calculator
- Choose metric or imperial units
- Enter your height
- Enter your weight
- Read your BMI and category — and read the caveats above before drawing conclusions
The categories
WHO classification for adults:
- Under 18.5 — underweight
- 18.5 – 24.9 — healthy range
- 25.0 – 29.9 — overweight
- 30.0 and above — obese
Thresholds are lower for South and East Asian populations, where metabolic risk rises at a lower BMI: many health bodies use 23 as the overweight threshold and 27.5 for obesity. India's national guidelines follow this.
What BMI misses
The formula uses only height and weight, so it cannot distinguish between kinds of mass or where it sits.
Muscle counts as weight. Athletes and heavily trained people routinely score as overweight or obese while carrying very little fat. The index is not measuring what it appears to.
Fat distribution is invisible to it. Visceral fat around the organs carries much higher metabolic risk than subcutaneous fat, and two people with identical BMI can differ enormously here. Waist circumference and waist-to-height ratio capture this better — a waist under half your height is a reasonable rule of thumb.
It is not valid for everyone. Children and teenagers need age- and sex-specific percentile charts. It is unreliable during pregnancy and in the elderly, where muscle loss changes the picture.
Use BMI as one rough signal among several, not as a verdict.
Frequently asked questions
Is BMI accurate for athletes?
No. Muscle is denser than fat, so muscular people frequently classify as overweight or obese despite low body fat. For anyone training seriously, body composition measurement is far more informative.
Why are the thresholds different for Asian populations?
Research consistently shows higher metabolic risk — type 2 diabetes and cardiovascular disease — at lower BMI values in South and East Asian populations. Many national guidelines use 23 as the overweight cutoff rather than 25.
Can I use this for my child?
No. Children's BMI must be interpreted against age- and sex-specific growth percentiles, because healthy body composition changes substantially through development. Use a paediatric chart or ask your doctor.
What is a better measure than BMI?
Waist-to-height ratio is a good, equally simple alternative and better reflects visceral fat — keep your waist under half your height. Body fat percentage, measured properly, is better still.