Body fat percentage is the fraction of the total body mass that is adipose tissue. This index is often used as a means to monitor progress during a diet or as a measure of physical fitness for certain sports, such as body building. It is more accurate as a measure of health than body mass index (BMI) since it directly measures body composition and there are separate body fat guidelines for men and women. However, its popularity is less than BMI because most of the techniques used to measure body fat percentage require equipment and skills that are not readily available.

Recommendations

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The American Council on Exercise^* has associated categories with ranges of body fat. Women generally have less muscle mass than men and therefore they have a higher body fat percentage range for each category.

Description	Women	Men
Essential fat	10-13%	2-5%
Athletes	14-20%	6-13%
Fitness	21-24%	14-17%
Acceptable	25-31%	18-24%
Obesity	32%+	25%+

Measurement techniques

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Dual Energy X-ray absorptiometry

Dual Energy X-ray absorptiometry, or DXA (formerly DEXA), is the most precise method for measuring body fat percentage.

Body average density measurement

Prior to the adoption of DXA, the most accurate method of determining body fat percentage was to weigh a person underwater in order to obtain the average density (mass per unit volume). Since fat tissue has a lower density than muscles and bones, it is possible to estimate the fat content. This estimate is distorted by the fact that muscles and bones have different densities: for a person with a more-than-average amount of bone tissue, the estimate will be too low. However, this method gives highly reproducible results for individual persons (+/- 1%), unlike the methods discussed below, which can have an error up to +/-10%.^* The body fat percentage is commonly calculated from one of two formulas:

• Brozek formula: BF = (4.57/ρ - 4.142) * 100
• Siri formula is: BF = (4.95/ρ - 4.50) *100

In these formulas, ρ is the body density in kg/L. For a more accurate measurement, the amount of bone tissue must be estimated with a separate procedure. In either case, the body density must be measured with a high accuracy. An error of just 0.2% (e.g. 150 mL of trapped air in the lungs) would make 1% difference in the body fat percentage. The body density is measured by weighing a person underwater, with all air expelled from the lungs. This procedure is normally carried out in laboratories with special equipment. However, it is possible to make an estimation without assistance in a swimming pool. A person who neither floats nor sinks with empty lungs in a swimming pool would have a density of 1 kg/dm³ and an estimated body fat percentage of 43% (Brozek) or 45% (Siri), which would be extremely obese. Persons with a lower body fat percentage would need to hold some kind of floatation device, such as an empty bottle, in order to prevent them from sinking. If the floatation device has mass m and volume v, and the person has a mass M, then his or her density is

$\backslash rho\; =\; \backslash frac\{\backslash rho\_w\}\{1\; +\; m/M\; -\; \backslash rho\_w\; v/M\},$

where $\backslash rho\_w$ is the density of water kg/L at 22 °C (72 °F). For example, a person weighing 80 kg needs to hold a floater with a volume of 4.5 L and a mass of 0.5 kg has a density of 1.05 kg/L and hence a body fat percentage of 21%. Note that both the Brozek and Siri formulas are claimed to give systematically too high body fat percentages. ^*

Skinfold test

A simpler test for measuring body fat is the skinfold test, in which a pinch of skin is precisely measured by calipers at standardized points on the body to determine the thickness of the subcutaneous fat layer, and converted to body fat percentage by an equation. This method is criticised as the accuracy is dependent on application of the test. There is room for inaccuracy dependent on who is applying the test. It can be useful as a measure in body composition change over a period of time (provided the test is carried out by the same person with the same technique) but will not give an accurate reading of body fat percentage.

Bioelectrical Impedance Analysis (BIA)

This method can be conducted accurately in the laboratory or at home with less accuracy, though this version is more affordable. The general principle behind BIA: two conductors are attached to a person's body and a small electrical charge is sent through the body. The resistance between the conductors will provide a measure of body fat, since the resistance to electricity varies between adipose, muscular and skeletal tissue. Criticism of this medthodolgy is based on where the conductors are placed on the body; typically they are placed on the feet, with the current sent up one leg, across the abdomen and down the other leg. As men and women store fat differently around the abdomen and thigh region, the results can be less accurate as a measure of total body fat percentage.

From weight and waist

Less accurate again, but most readily available (e.g. on the internet), are formulae which calculate a percentage from weight, waist measurement and sometimes other measurements. These are widely available on the internet and elsewhere.

External links

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• Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index PMID 10966886
• U.S. Navy Circumference Method
• BMI to Body Fat formula
• Calculator
• Body Fat Estimator, based on Ponderal Index or Waist size
• Body Fat Calculators Selection of different body fat calculators based on BMI, skinfolds and anatomical measures.
• Yuhasz Skinfold Test

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