Definition of Obesity

Webster’s dictionary defines obesity as “a condition characterized by the excessive accumulation and storage of fat in the body.”

The Obesity Medicine Association’s definition of obesity is “a chronic, relapsing, multifactorial, neurobehavioral disease, wherein an increase in body fat promotes adipose tissue dysfunction and abnormal fat mass physical forces, resulting in adverse metabolic, biomechanical, and psychosocial health consequences.”

There are a number of methods to quantitatively define and categorize obesity.

Body Mass Index

Body mass index (BMI), used by the Centers for Disease Control and Prevention and the World Health Organization, is a person's weight in kilograms divided by their height squared in meters. BMI often correlates with the degree of body “fatness” but should not be the sole indicator for presence of metabolic disease risk. Advantages of using BMI include being easy to perform, low cost, and reproducible. Disadvantages include not accounting for gender, ethnicity, muscle mass, and frame size.

BMI may be used to categorize weight.

  • Underweight: ≤ 18.5 kg/m²
  • Normal weight: 18.5-24.9 kg/m²
  • Overweight: 25.0-29.9 kg/m²
  • Class I Obesity: 30.0-34.9 kg/m²
  • Class II Obesity: 35.0-39.9 kg/m²
  • Class III Obesity: ≥ 40.0 kg/m²*

*Class III obesity is sometimes categorized as “extreme” or “severe” obesity.

Abdominal Circumference

Abdominal circumference is a measurement of central adiposity and correlates well with metabolic disease risk. It is easy to perform and inexpensive. Results may vary due to measurement technique. There are also racial and ethnic differences in the cut-off points for obesity with abdominal circumference.

  • Caucasian Males: ≥ 40 inches correlates with obesity
  • Caucasian Females: ≥ 35 inches correlates with obesity
  • Asian Males: ≥ 35 inches correlates
  • Asian Females: ≥ 31 inches correlates with obesity

Body Fat Percentage

Body fat percentage is the most accurate method to assess adiposity. Measurements are obtained with DEXA scanning, air or water displacement, 4-point or 2-point bioimpedance, or skin calipers, in decreasing order of accuracy.

  • Males: Obesity ≥ 25% body fat
  • Females: Obesity ≥ 32% body fat

Body composition testing accounts for differences in age, gender, body frame size, and muscle mass. Testing can be more expensive to perform, have limited availability, and is more limited in assessing for metabolic disease risk than abdominal circumference.

In summary, obesity can be quantitatively assessed using any of the three methods discussed. The pros and cons of each type of measurement, as well as the type of information desired, should be considered when assessing for obesity.


This article on the definition of obesity was written by Anna Welcome, MD, FACOG. Dr. Welcome is a medical bariatrician at Eviva in Edmonds, Washington.