The Center for Disease Control defines Body Mass Index (BMI) as “a number calculated from a person’s weight and height. BMI provides a reliable indicator of body fatness for most people and is used to screen for weight categories that may lead to health problems”. Body Mass Index does not measure body fat directly. Instead, it is an easy-to-perform, inexpensive screening and research shows it is accurate. Because it is a screening tool and not a diagnostic tool it will only identify possible weight problems. The Body Mass Index measures our body fat from underweight to obese using the formula to screen both children and adults. A healthcare provider would perform further assessments if they suspected excess weight to be a health risk.
Initially, the Quetelet Index, used in the 19th Century was used to determine body fat percentages. However, in 1972 Ancel Keys a paper was published in the Journal of Chronic Diseases which found the best proxy to be the BMI for population studies and not individual diagnosis. We adopted it as a standard formula to measure body fat because it was so easy to use. This formula is controversial because people, including physicians, have come to rely on it’s authority for medical diagnosis. It is intended purpose was meant to classify sedentary individuals or populations with an average body composition. The formula is a person’s body mass divided by the square of their height in metric units.
For a given mass, the BMI is inversely proportional; for taller people, however, this is skewed. For these reasons it can be controversial and not truly accurate. Childrens BMI are calculated the same way although the formula takes into consideration a percentile that compares children of the same sex and age. If you have little ones, you may hear the pediatrician tell you your child is in the 65th percentile, for example.
Bottom line: if you use it as an easy screening to be a guide, it can be helpful; but, if you are going to deem it entirely accurate, it is not.