A healthy body mass index is between 18.5 and 24.9 kg/m2 for adults 20 years or older. BMI is calculated using height and weight and is used as a predictor of body fat. The National Institute of Health provides a BMI calculator on its website.
A BMI under 18.5 indicates an individual is underweight, a value above 25 is overweight, and 30 or above is obese. BMI is used to determine possible risk for health problems due to obesity. Since it does not take into account body weight differences due to muscle or bone density, it is not as accurate for certain populations, such as athletes, the elderly or children, according to The Livestrong Foundation.