Using Body Mass Index
The most common way to determine if a person is overweight or obese is to calculate body mass index (BMI). BMI is an estimate of body fat based on comparing a person’s weight to his or her height.
Health care providers also look at BMI, along with information about additional risk factors, to determine a person’s risk for developing weight-related diseases. Usually, the higher a person’s BMI, the higher the risk of disease.
BMI for Adults
An adult’s BMI can be determined using a BMI calculator. Health care providers use BMI ranges to indicate a person's weight status. For adults, a BMI of:
- 18.5 to 24.9 is considered normal weight
- 25.0 to 29.9 is considered overweight
- 30.0 to 39.9 is considered obese
- 40.0 and higher is considered extremely obese1
It is important to remember that although BMI is generally a good way to estimate how much body fat a person has, it does not measure body fat directly and therefore is not reliable in all cases. For example, a person may weigh extra because he or she is athletic and has a lot of muscle, and not because he or she has excess body fat.
BMI for Children and Teens
For children age 2 and older and for teens, BMI uses weight and height, but adds sex and age into the calculation. Instead of using a specific number like the BMI charts for adults, the BMI for children and teens is listed as a percent. This percentage indicates a child’s BMI in relation to the BMIs of other children of the same sex and age. A child and teen BMI calculator can provide a BMI.
Children age 2 and older are considered:
- At a healthy weight if their BMI falls between the 5th and the 85th percentiles
- Overweight if their BMI is between the 85th and 95th percentiles
- Obese if their BMI is at or higher than the 95th percentile2
Other Ways to Measure Body Fat
Body fat can be measured in other ways in addition to BMI, including waist circumference, calculation of waist-to-hip circumference ratios, measuring the thickness of a skinfold (a pinch of skin and fat), and techniques such as ultrasound that are more precise than BMI. A health care provider can help determine if such tests are necessary.
- National Heart, Lung, and Blood Institute. (2012). How are overweight and obesity diagnosed? Retrieved August 8, 2012, from https://www.nhlbi.nih.gov/health/educational/lose_wt/risk.htm
- Barlow, S. E., & the Expert Committee. (2007). Expert committee recommendations regarding the prevention, assessment, and treatment of child and adolescent overweight and obesity: Summary report. Pediatrics, 120, S164–S192.