BMI
Also called: body mass index
Clinical reviewer: Jacob Moore, MD · Last checked September 24, 2026
What is BMI?
BMI (body mass index) is a measurement that uses weight and height to estimate overweight and obesity, calculated as weight in kilograms divided by height in meters squared. In adults, the World Health Organization defines overweight as a BMI of 25 or greater and obesity as a BMI of 30 or greater. BMI is a surrogate marker of fatness.
What are the adult BMI categories?
BMI is a measurement that uses weight and height to estimate overweight and obesity, calculated as weight in kilograms divided by height in meters squared [1][2]. For adults age 20 and older, NIDDK uses these categories [2]:
| BMI | Category |
|---|---|
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 or higher | Obesity |
| 40 or higher | Severe obesity |
NIDDK says weight-management medications may be prescribed for adults with a BMI of 30 or greater, or 27 or greater with weight-related health problems [4].
What are BMI's limits as a health measure?
BMI is a surrogate marker of fatness, and additional measurements, such as waist circumference, can help the diagnosis of obesity [1]. A 2025 Lancet Commission wrote that current BMI-based measures of obesity can both underestimate and overestimate adiposity and provide inadequate information about health at the individual level, and recommended BMI be used only as a surrogate measure of health risk at a population level, for epidemiological studies, or for screening, rather than as an individual measure of health [3]. In people with a BMI above 40, the Commission wrote that excess adiposity can pragmatically be assumed [3].
Related terms
Sources
This entry is for education and is not medical advice. Talk with a licensed clinician about your own situation before starting, stopping, or changing any treatment.
