BMI Explained
Body mass index, or BMI, is a quick number that compares your weight to your height. Doctors and health sites use it as a rough screen because it is easy to calculate and works reasonably well across large groups of people. It is a starting point, not a full picture of health.
The BMI formula
BMI = weight in kilograms divided by height in metres squared. Someone 1.75 m tall weighing 70 kg has a BMI of 70 / (1.75 x 1.75), which is about 22.9.
The BMI categories
- Under 18.5: underweight
- 18.5 to 24.9: normal or healthy range
- 25 to 29.9: overweight
- 30 and above: obese
A BMI of 22.9 sits comfortably in the healthy range.
Why BMI is only a starting point
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 is stored, and body fat around the waist carries more health risk than fat elsewhere. Two people with the same BMI can be in very different shape.
What to look at alongside it
Waist measurement, body fat percentage and how you feel and move all add context BMI misses. BMI is a useful signal, not a diagnosis, and it is no substitute for advice from a healthcare professional.
Check yours
Use the BMI Calculator for your number, then the Ideal Weight Calculator and Body Fat Calculator for a fuller view.
Frequently asked questions
Does BMI use age?
No. Adult BMI uses only weight and height. Age is used for children's BMI-for-age percentiles, not for adults.
Is BMI accurate for muscular people?
Not always. Muscle weighs more than fat, so athletes can read as overweight while being lean. Use waist size and body fat alongside it.
What is a healthy BMI range?
For most adults, 18.5 to 24.9 is considered healthy. It is a general guide, not a personal diagnosis.