A healthy body mass index for an adult is between 18.5 and 24.9. Below this level is considered underweight, and above this level is considered overweight. A BMI of 30 or higher is considered obese, according to WebMD.
These numbers are only a guide. This tool is helpful in determining if weight is increasing, or if there is a risk for disease. People who are very muscular may not get an accurate reading using only their height and weight. A person who experiences a large change in BMI should be evaluated to understand the cause. To determine obesity, a waist circumference is also necessary, according to WebMD.