Medical professionals will classify you as underweight if your body mass index (BMI) is less than 18.5. If your body mass index (BMI) is between 18.5 and 24.99, you are regarded to have an average body mass index or be within the Healthy Weight range. If your body mass index (BMI) falls between 25.0 and 29.9, you are regarded to have an unhealthy level of excess fat. No matter how old you are, if your body mass index (BMI) is 30.0 or greater, you are deemed obese.