The most commonly used method of defining levels of obesity is the body mass index (BMI). BMI is defined as your body mass in kilograms divided by the square of your height in metres. If you have a BMI over 30 you are considered obese. If your BMI is greater than 35 you have severe obesity. Morbidly obese patients have a BMI greater than 40.
There are limitations to the use of BMI to estimate body fat. BMI overestimates body fat in people with more lean body mass (eg athletes) and underestimates body fat on those with less lean body mass (eg elderly). Despite this BMI remains the most commonly used measure to assess body fat.