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, and the US Centers for Disease Control describes it as a screening tool rather than a diagnosis.
The BMI formula (metric)
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 formula (imperial)
If you measure in pounds and inches, multiply by 703: BMI = weight in pounds / (height in inches squared) x 703. Someone 5 foot 9 (69 inches) weighing 155 lb has a BMI of 155 / (69 x 69) x 703, which is also about 22.9. The two formulas give the same answer; only the units differ.
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. These bands are the same for adult men and women.
Is BMI calculated differently for children?
Yes. For children and teenagers, the same weight-and-height number is read against age- and sex-specific percentile charts, not the fixed adult bands above, because a healthy body changes shape as a child grows. So a child's BMI only means something next to those percentiles.
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. A common rule of thumb is to keep your waist under half your height. 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
How do I calculate BMI?
In metric, divide weight in kg by height in metres squared: 70 / (1.75 x 1.75) is about 22.9. In imperial, use weight in pounds / (height in inches squared) x 703, which gives the same result.
Does BMI use age?
No, for adults. Adult BMI uses only weight and height. Age and sex are used for children's BMI-for-age percentiles, not for adults.
Is BMI accurate for muscular people?
Not always. Muscle is denser 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, the same for men and women. It is a general screen, not a personal diagnosis, and does not apply to children.