BMI is calculated using conventional weight status categories for people aged 20 and up. These categories apply to men and women of all ages and physical shapes. Underweight is defined as a body mass index (BMI) of less than 18.5 Healthy Weight is between 18.5 and 24.9 pounds. Overweight (BMI of 25.0–29.9) Obesity is defined as a body mass index of 30.0 or higher.