Obesity is defined by the Centers for Disease Control and Prevention as anyone who has a Body Mass Index (BMI) over 30.
To calculate your BMI, divide your weight in pounds by your height in inches, squared, then multiply that number by 703:
weight (lb) / [height (in)]² x 703
The CDC associates the following weight categories with BMI numbers:
For some people, like athletes, high BMIs may not indicate that they are at unhealthy weights despite being in an unhealthy range, since BMI numbers don't directly measure body fat.