For adults 20 years and older, a healthy body mass index, or BMI, is between 18.5 and 24.9, according to WebMD. Any BMI below 18.5 is considered underweight. BMIs between 25 and 29.9 are considered overweight. BMIs above 30 are considered obese.
The BMI calculation is based on an average adult's height and weight. The BMI scale is not necessarily a determination of health, however. For example, if a person is very muscular or very frail, the BMI may attribute that person to be overweight or underweight respectively. Clinical diagnoses for obesity also use waist circumference for a more accurate determination of adult obesity.