How BMI is calculated
Body mass index is a quick screen that compares your weight to your height. In metric units it is your weight in kilograms divided by your height in metres squared. It does not measure body fat directly, but for most adults it gives a useful starting category.
The measure was devised by the Belgian mathematician Adolphe Quetelet in 1832 (originally the Quetelet Index), and the name "body mass index" was coined by the American scientist Ancel Keys in 1972. It uses only weight and height, not age.
What is the BMI formula?
BMI = weight (kg) / height (m) squared. For example, someone 1.75 m tall weighing 70 kg has a BMI of 70 / (1.75 × 1.75), which is about 22.9.
What do the BMI categories mean?
Under 18.5 is underweight, 18.5 to 24.9 is the normal range, 25 to 29.9 is overweight, and 30 or above sits in the obese range. A BMI of 22.9 falls comfortably in the normal band.
Where does BMI fall short?
BMI cannot tell muscle from fat, so very muscular people can read as overweight while carrying little fat. It also ignores age, sex and where fat sits on the body. Treat it as one simple signal to start from, not a diagnosis, and read it alongside other measures like waist size.
Frequently asked questions
Does BMI use your age?
No. Adult BMI is worked out from weight and height only. Age is used for children's BMI-for-age percentiles and for calorie needs, but not for standard adult BMI.
Can I use pounds and feet?
Yes. Switch the unit dropdowns to pounds or stone for weight, and feet or inches for height. You get the same BMI whichever units you use.
Is BMI accurate for athletes?
Not always. Muscle weighs more than fat, so muscular people can show a high BMI while being lean. Waist size and body fat percentage give a fuller picture.
What is a healthy BMI range?
For most adults, a BMI between 18.5 and 24.9 is considered the healthy range. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is in the obese range. For example, someone 1.75 m tall weighing 70 kg has a BMI of about 22.9, which sits comfortably in the healthy band. Treat these figures as a general screen rather than a personal diagnosis: very muscular people can read high, and the ranges do not account for age, sex or where fat is carried on the body.
Does BMI work for children?
Children use age- and sex-specific percentiles, not the adult bands. This calculator is intended for adults.
What about waist size?
Waist circumference and waist-to-height ratio add useful information about health risk alongside BMI.