How to Calculate BMI and What Your Number Means
Understand the BMI formula for both metric and imperial units, what the standard categories mean, and the important limitations of body mass index.
Body Mass Index (BMI) is a widely used screening number that estimates whether a person's weight is in a healthy range relative to their height. It is quick to calculate and useful as a general indicator, though it is not a complete measure of health.
The BMI Formula
BMI divides weight by the square of height. The metric and imperial versions look like this:
Metric: BMI = weight(kg) / height(m)^2
Imperial: BMI = 703 x weight(lb) / height(in)^2
Example: 70 kg, 1.75 m tall
70 / (1.75 x 1.75) = 22.9Standard BMI Categories
For most adults, the World Health Organization uses these ranges:
- Below 18.5: underweight
- 18.5 to 24.9: normal weight
- 25.0 to 29.9: overweight
- 30.0 and above: obese
Important Limitations
BMI is a rough screening tool, not a diagnosis. Because it only uses height and weight, it cannot distinguish muscle from fat. Athletes with high muscle mass may register as overweight, while BMI can also miss unhealthy fat distribution in others. It is also less reliable for children, older adults, and pregnant people.
Treat BMI as a starting point for a conversation, not a verdict. For personal health decisions, consult a qualified healthcare professional who can consider your full picture.
To check your number in either unit system instantly, use the BMI Calculator. It runs entirely in your browser, so your height and weight are never sent anywhere.