The body adiposity index (BAI) is a more recently developed alternative to the body mass index (BMI) that is purported by its developers to be a better measure of body fat in individuals than BMI. The formula divides the circumference around the widest part of the hips and buttocks in centimeters by the height in meters times the square root of the height in meters, minus 18. This is in contrast to the equation for BMI, which simply takes the weight in kilograms divided by the height in meters squared.
A main argument against using a BMI measurement to estimate whether an individual is overweight is that the BMI equation only takes into account a person's height and weight. This measure of body fat has long been criticized for inaccuracy, especially when it comes to individuals with a large amount of muscle mass or very little muscle mass. For example, a 150 pound (68 kg) woman who is five feet, six inches tall (1.67 meters) with very little muscle mass and a high percentage of body fat would have the same BMI as a 150 pound (68 kg) muscular woman of the same height, who has a much lower percentage of body fat. While they are clearly at different fitness levels, the BMI measurement would judge them to be the same.
The body adiposity index, however, takes into account the height and the hip circumference of the individual. It is very unlikely, following in the same example, that both 150 pound women have the exact same hip circumference. Since muscle weighs much more than the same volume of fat, the more muscular woman will be more likely to have a smaller hip circumference, because she has less fat on her body than the other woman. The body adiposity index would reflect this difference, where the BMI fell short.
One possible shortcoming of the body adiposity index, according to fitness experts, is that it was developed and has been primarily tested on Hispanic-Americans and African-Americans, who have different body fat distributions than Americans of European descent and other populations. Therefore, the hip measurement, which was found to be an accurate predictor of the amount of body fat among Hispanic-Americans and African-Americans, may not work as well on people from other populations who may carry weight higher on the abdomen, lower on the thighs, or elsewhere. The body adiposity index may also not be accurate in children, who carry body fat much differently than adults do.