What is BMI?
Body Mass Index (BMI) is a simple number that relates your weight to your height. It is widely used as a quick screening tool to place an adult into a weight category — underweight, normal, overweight or obese. Because it needs only two measurements, it is the most common starting point for thinking about a healthy weight.
How BMI is calculated
In metric units, BMI is your weight in kilograms divided by the square of your height in metres. In imperial units, it is your weight in pounds times 703, divided by the square of your height in inches. This calculator handles both, so you can use whichever you are comfortable with.
The categories
- Under 18.5 — underweight
- 18.5 to 24.9 — normal weight
- 25 to 29.9 — overweight
- 30 and above — obese
What BMI does and doesn't tell you
BMI is a useful rough guide but it is not a measure of body fat or health on its own. It cannot tell muscle from fat, so very muscular people can score as overweight while having low body fat. It also does not account for age, sex or where fat is stored. Treat it as one signal among several, and talk to a healthcare professional for a fuller picture.
Frequently asked questions
Is BMI accurate for everyone?
It is a general guide for adults. Athletes, older people, pregnant women and children may need different measures.
Which units should I use?
Either — pick metric or imperial and enter both values in those units.
Is my data private?
Yes. Everything is calculated in your browser; nothing is saved or sent anywhere.