An adult body mass index (BMI) chart uses an individual's height and weight to calculate a BMI number, states the American Cancer Society. This number determines whether the individual is at a healthy weight, underweight, overweight or obese. A BMI less than 18.5 is considered underweight, a number between 18.5 and 24.9 is normal weight; anything between 25 and 29.9 is overweight and a number larger than 29.9 is obese.
Although BMI charts can be helpful in determining if an individual is at a healthy weight, they are not accurate in every situation. Athletes or those who participate in regular weight lifting may have a large amount of muscle mass that puts them in the overweight or obese category, even though they have a healthy body fat percentage.
Those with a small amount of muscle mass may fall into the healthy weight category, even when their body fat percentage is at an unhealthy level, notes the American Cancer Society.