One assumption is that individuals who have a BMI within the normal range have an average amount of body fat. The other is that every ounce of body weight over the standard weight for height is fat. These assumptions generate the most classification errors for children with BMI values in the gray area between normal weight and overweight, which is a BMI between 18 and 20 for most ages, Dr. Ellis says. The study found that body fat in children in this BMI range varied from 10 to 40 percent. Males with body-fat levels over 25 percent and females with levels over 30 percent are generally considered obese.
Few people think that normal-weight kids can have too much body fat.
High levels of physical activity, early maturation, genetics and ethnicity can all contribute to a child having a high BMI but a healthy amount of body fat.