A person’s BMI, or Body Mass Index, is used to determine what percentage of an average person’s body weight is muscle and what percentage is fat. In the majority of cases, this is an accurate calculation. It is used to help a person reach an ideal weight if they are embarking on a weight loss program. Men and women will have different percentages of fat, according to their natural physical makeup. Women tend to carry more fat and will have a somewhat higher BMI. There are times when a person’s BMI can be misleading, however.