BMI, or Body Mass Index, is the number most of us meet at a doctor's visit. It is weight divided by height squared, and it has been used for over a century as a quick health screening tool.
How it is calculated
In metric: BMI = weight (kg) ÷ height (m)². In imperial: 703 × weight (lb) ÷ height (in)². The WHO categories are: underweight below 18.5, normal 18.5–24.9, overweight 25–29.9, and obese 30 and above.
What it measures — and what it misses
BMI correlates with body mass, not body composition. It cannot tell muscle from fat. A rugby player and an office worker of the same height and weight get the same BMI, despite very different metabolisms. It also makes no allowance for age, sex composition or where fat sits on the body.
Use it as a trend, not a verdict
The most honest use of BMI is longitudinal: watching the direction of the number over months. Combined with a waist measurement and how you feel, it is a reasonable screening signal. The error is treating a snapshot as a diagnosis.
Our BMI calculator returns your number and category in seconds, accepting either metric or imperial units. For a more complete picture, pair it with the calorie and macro calculators — but remember the numbers are estimates, not prescriptions.