The normal body mass index for adults aged 20 and older is 18.5 to 24.9, notes the Centers for Disease Control and Prevention. A BMI below 18.5 determines that a person is underweight, while a BMI of 25.0 to 29.9 determines a person is overweight.
Body mass index establishes whether a person is at a healthy weight for his height, explains the American Cancer Society. To calculate the BMI, an individual can divide his weight in kilograms by the square of his height in meters. Children and teens can calculate their BMI in the same way, but ranges are different because body fat changes based on age. Additional ways to calculate BMI include online calculators and tables.