The body fat percentage (BFP) of a human or other living being is the total mass of fat divided by total body mass, multiplied by 100; body fat includes essential body fat and storage body fat. Essential body fat is necessary to maintain life and reproductive functions. The percentage of essential body fat for women is greater than that for men, due to the demands of childbearing and other hormonal functions. Storage body fat consists of fat accumulation in adipose tissue, part of which protects internal organs in the chest and abdomen. A number of methods are available for determining body fat percentage, such as measurement with calipers or through the use of bioelectrical impedance analysis. The body fat percentage is a measure of fitness level, since it is the only body measurement which directly calculates a person's relative body composition without regard to height or weight. The widely used body mass index (BMI) provides a measure that allows the comparison of the adiposity of individuals of different heights and weights. While BMI largely increases as adiposity increases, due to differences in body composition, other indicators of body fat give more accurate results; for example, individuals with greater muscle mass or larger bones will have higher BMIs. As such, BMI is a useful indicator of overall fitness for a large group of people, but a poor tool for determining the health of an individual. Typical body fat amounts Epidemiologically, the percentage of body fat in an individual varies according to sex and age. Various theoretical approaches exist on the relationships between body fat percentage, health, athletic capacity, etc. Different authorities have consequently developed different recommendations for ideal body fat percentages. This graph from the National Health and Nutrition Examination Survey (NHANES) in the United States charts the average body fat percentages of Americans from samples from 1999 to 2004: In males, mean percentage body fat ranged from 23% at age 16–19 years to 31% at age 60–79 years. In females, mean percentage body fat ranged from 32% at age 8–11 years to 42% at age 60–79 years. But it is important to recognise that women need at least 9% more body fat than men to live a normal healthy life. Data from the 2003–2006 NHANES survey showed that fewer than 10% of American adults had a "normal" body fat percentage (defined as 5–20% for men and 8–30% for women). Results