Body mass index (BMI)
Body weight relative to height squared, given in kg/m²
Also known as
Definition
The body mass index (BMI) relates body weight to height: weight in kilograms divided by height in meters squared. At 70 kg and 1.75 m, the BMI is about 22.9 kg/m². The WHO uses the BMI to classify overweight and obesity in adults. It is a surrogate marker for body fat: it does not measure fat directly and cannot tell fat apart from muscle, bone and water.
| Parameter | Value |
|---|---|
| Unit | kg/m² |
| Reference Range | Adults: 18.5–24.9 kg/m² is considered a healthy weight, below 18.5 underweight, 25 or above overweight and 30 or above obesity (class 1: 30–34.9; class 2: 35–39.9; class 3: 40 or above). Children and teens are assessed with age- and sex-specific percentiles. Discuss how your value fits with your other findings with your doctor. |
Agree on your personal target range with your doctor.
With pounds and inches: weight (lb) × 703, divided by height (in) squared. The result is the same BMI in kg/m².
↓ What a low value means
A BMI below 18.5 kg/m² is classified as underweight in adults. For older adults the picture differs: according to MedlinePlus, from age 65 a slightly higher BMI may help protect against bone thinning (osteoporosis), and a normal BMI can go along with less muscle mass. Discuss weight changes with your doctor before you try to gain or lose weight.
↑ What a high value means
According to the WHO, a BMI of 25 kg/m² or above is overweight and 30 or above is obesity. In most adults a higher BMI goes along with more body fat and a higher risk of heart and blood vessel disease, high blood pressure, type 2 diabetes, high blood lipids, fatty liver disease (MASLD), arthritis in the knees and hips, sleep apnea and certain cancers, among others. But the BMI cannot tell fat from muscle: very muscular people, for example in strength sports, can have a BMI in the overweight range without carrying too much body fat. Waist circumference adds to the picture because the BMI does not show where fat is stored.
When to test
The BMI is calculated from height and weight and is often part of a checkup; MedlinePlus states at least once a year for most adults and for children from age 2. It is also used to follow weight changes over time. Children and teens are assessed with their own percentile charts.
Frequently asked questions
Is the BMI meaningful for athletes? +
Only to a limited extent. The BMI cannot tell whether weight comes from fat or muscle. Very muscular people can therefore have a BMI in the overweight range without carrying too much body fat. Waist circumference or other measurements can add to the picture.
Can you have too much body fat with a normal BMI? +
Yes. According to MedlinePlus this is more common in groups that tend to have lower BMI values, such as people of Asian descent and older adults who have lost muscle. The BMI also does not show where in the body fat is stored.
Do the BMI categories apply to children? +
No. For children from age 2 and teens, the BMI is compared with growth charts for age and sex. A healthy weight is usually the 5th to 85th percentile, overweight the 85th to 94th, and obesity the 95th percentile or above.
What does waist circumference add? +
It estimates fat around the abdomen. According to MedlinePlus, a lot of abdominal fat is linked to a higher risk of related conditions even with a BMI in the healthy range. That is why waist circumference is often measured as well.
Body mass index (BMI)
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.