“Obesity” is a medical diagnosis based on your body mass index (BMI). In general, here are the medical definitions we use for kids:

  • Underweight: BMI <5th percentile (based on age and sex)
  • Overweight: BMI between the 85th and 95th percentile (based on age and sex)
  • Obese: BMI ≥95th percentile (based on age and sex)

However, BMI as a measure of health is very problematic and is an oversimplification. The calculation of BMI only takes weight and height into account; it does not include the percent body fat or muscle mass. As a result, someone who is a coach potato and weighs 300 pounds has the same BMI as a professional basketball player who weighs 300 pounds (if they are the same height).

Generally, we mean “obese” to mean someone who has BMI ≥95th percentile and most of their weight is from fat tissue (as opposed to muscle), but we do not actually measure that.

The more important thing than categorizing someone as underweight, overweight, or obese is understanding their lifestyle and working to create the healthiest lifestyle possible in terms of diet and exercise.