According to Thomas A. Owens, M.D. (Departments of Internal Medicine and Pediatrics, Duke University Medical Center, Durham, NC), body fat percentage is categorized as follows:
|Adults in United States, average||22-25%||15–19%|
According to Health Check Systems, The American Council on Exercise has categorized ranges of body fat percentages as follows:
Two different types of X-ray scans the body, one that detects all tissues and another that doesn't detect fat. A computer can subtract the second picture from the first one, giving only fat detection. The mass of this can be estimated by the grade of exposure.
In addition, the most refined method, in-vivo neutron activation, can quantify all the elements of the body and use mathematical relations among the measured elements in the different components of the body (fat, water, protein, etc.) to develop simultaneous equations to estimate total body composition, including body fat. This is the most accurate method.
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 uncertainty up to ±10%. The body fat percentage is commonly calculated from one of two formulas:
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.
One way to determine body density is by hydrostatic weighting, which refers to measuring the apparent weight of a subject under water, with all air expelled from the lungs. Since approximately 1.2 L of the air in the lungs cannot be expelled, the formulas require an adjustment described below. This procedure is normally carried out in laboratories with special equipment.
The weight that is thus found will be equivalent to the body's weight in air, minus the weight of the volume of water which that object displaces. The following formula can be used to compute the relative density of a body: its density relative to the liquid in which it is immersed, based on its weight in that liquid:
Note that it is unnecessary to actually weigh a body under water in order to determine its volume, density or, for that matter, its weight under water. Volume can be easily determined by measuring how much water is displaced by submerging that body.
For a human body, a vertical tank which has a uniform cross-section-area, such as a cylinder or prism, can be used. As the subject submerges and expels air from the lungs, the rise in the water level is measured. The water level rise, together with the interior dimensions of the tank, determine the displaced volume. An adjustment for air remaining in the lungs (about 1.2 liters after full exhalation) is requried.
It is possible to obtain an estimate of body density without directly measuring weight underwater, and without directly measuring water displacement. What is required is a swimming pool or other tank where the subject can be fully immersed. The idea is to balance the body with a buoyant floatation device of a suitable mass and volume, such that the body plus floatation device neither sink nor float. The viability of this method rests in choosing a floatation device which has some convenient attribute that makes it possible to determine its volume easily: it is small, regularly shaped, and perhaps manufactured to a specific volume. From the volume and mass of the balancing floatation device, the mass of the body, and a 1.2 liter adjustment for air still in the lungs after full exhaling, the volume and density of the body can be determined.
A person who neither floats nor sinks with empty lungs in water would have a density of approximately 1 kg/L (the density of water) 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 keep 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
A simpler version of the above formula can be derived by making two assumptions, and one small algebraic change. Firstly, the density of water can be taken to be 1 kg/L, which is very accurate. Secondly, the mass of light floatation device such as an empty plastic bottle is very small and so the term is negligible: if this assumption is invalid, it can easily be compensated for, as described below. Thirdly, the numerator and denominator can be multiplied by M, finally yielding
A light plastic bottle filled with air makes a convenient floater, since the amount of air in it can be adjusted and accurately measured. The plastic occupies only a small volume is about the same density as water so that there is very little error in not correcting for the plastic bottle. The measurement begins with the bottle completely empty. The subject is asked to expel as much as as possible from the lungs and use the bottle as a completely submerged floater. Water is allowed to enter the submerged bottle until the person sinks beneath the surface without touching the bottom. The liters of air in the container is equal to the kg of water displaced. is equal to this kg plus the estimated 1.2 kg of water displaced by the air remaining in the lungs after full exhalation.
There exist various anthropometric methods for estimating body fat. The term anthropometric refers to measurements made of various parameters of the human body, such as circumferences of various body parts or thicknesses of skinfolds. Most of these methods are based on a statistical model. Some measurements are selected, and are applied to a population sample. For each individual in the sample, the method's measurements are recorded, and that individual's body density is also recorded, being determined by, for instance, under-water weighing, in combination with a multi-compartment body density model. From this data, a formula relating the body measurements to density is developed.
Because most anthropomorphic formulas such as the Durnin-Womersley skinfold method, the Jackson-Pollock skinfold method, and the US Navy circumference method, actually estimate body density, not body fat percentage, the body fat percentage is obtained by applying a second formula, such as the Siri or Brozek described in the above section on density. Consequently, the body fat percentage calculated from skin folds carries the cumulative error from the application of two separate statistical models.
These methods are therefore inferior to a direct measurement of body density and the application of just one formula to estimate body fat percentage. One way to regard these methods is that they trade accuracy for convenience, since it is much more convenient to take a few body measurements than to submerge individuals in water tanks.
The chief problem with all statistically derived formulas is that in order to be widely applicable, they must be based on a broad sample of individuals. Yet, that breadth makes them inherently inaccurate. The ideal statistical estimation method for an individual is based on a sample of similar individuals. For instance, a skinfold based body density formula developed from a sample of male collegiate rowers is likely to be much more accurate for estimating the body density of a male collegiate rower than a method developed using a sample of the general population, because the sample is narrowed down by age, sex, physical fitness level, type of sport, and lifestyle factors. On the other hand, such a formula is unsuitable for general use.
The skinfold estimation methods are based on a skinfold test, whereby a pinch of skin is precisely measured by calipers at several standardized points on the body to determine the subcutaneous fat layer thickness. These measurements are converted to an estimated body fat percentage by an equation. Some formulas require as few as three measurements, others as many as seven. The accuracy of these estimates is more dependent on a person's unique body fat distribution than on the number of sites measured. As well, it is of utmost importance to test in a precise location with a fixed pressure. Although it may not give an accurate reading of real body fat percentage, it is a reliable measure of body composition change over a period of time, provided the test is carried out by the same person with the same technique.
Skinfold-based body fat estimation is sensitive to the type of caliper used, and technique. This method also only measures one type of fat: subcutaneous adipose tissue (fat under the skin). Two individuals might have nearly identical measurements at all of the skin fold sites, yet differ greatly in their body fat levels due to differences in other body fat deposits such as visceral adipose tissue: fat in the abdominal cavity. Some models partially address this problem by including age as a variable in the statistics and the resulting formula. Older individuals are found to have a lower body density for the same skinfold measurements, which is assumed to signify a higher body fat percentage. However, older, highly athletic individuals might not fit this assumption, causing the formulas to underestimate their body density.
There also exist formulas for estimating body fat percentage from an individual's weight and girth measurements. For example, the U.S. Navy Circumference method compares abdomen or waist and hips measurements to neck measurement and height, and other sites claim to estimate one's body fat percentage by a conversion from the body mass index. In the Navy the method is known as the "rope and choke." It is not uncommon for Chiefs to take advantage of flaws in the procedure to pass those who are otherwise out of standards.
The U.S. Marine Corps and U.S. Army also rely on the Height and Circumference method. For males, they measure the neck and waist just above the navel. Females are measured around the hips, waist, and neck. These measurements are compared to a height/weight chart with age factored in as well. This method is used because it is a cheap and convenient way to implement a body fat test throughout the entire Department of Defense. This method poses a particular threat of inaccuracy because one can hold one's stomach in more if needed to pass the requirements, and/or flare the neck out and make it bigger resulting in a lower body fat percentage.
Due to different body compositions, those with larger necks have an advantage over those with smaller necks.
Another well-known method using height and circumference is the YMCA formula. It uses only body weight in pounds, and waist in inches (at navel), to calculate body fat percentage using the formulas,