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 or other anthropometric methods 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. 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. body fat caliper skinfold methods the skinfold estimation methods are based on a skinfold test, also known as a pinch test, whereby a pinch of skin is precisely measured by calipers, also known as a plicometer, 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. ultrasound ultrasound is used extensively to measure tissue structure and has proven to be an accurate technique to measure subcutaneous fat thickness. a-mode and b-mode ultrasound systems are now used and both rely on using tabulated values of tissue sound speed and automated signal analysis to determine fat thickness. by making thickness measurements at multiple sites on the body you can calculate the estimated body fat percentage. ultrasound techniques can also be used to directly measure muscle thickness and quantify intramuscular fat. ultrasound equipment is expensive, and not cost-effective solely for body fat measurement, but where equipment is available, as in hospitals, the extra cost for the capability to measure body fat is minimal. height and circumference methods main article: body mass index 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 u.s. navy, the method is known as the "rope and choke." there is limited information, however, on the validity of the "rope and choke" method because of its universal acceptance as inaccurate and easily falsified. the u.s. army and u.s. marine corps 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 then looked up in published tables, with the individual's height as an additional parameter. this method is used because it is a cheap and convenient way to implement a body fat test throughout an entire service. methods using circumference have little acceptance outside the department of defense due to their negative reputation in comparison to other methods. the method's accuracy becomes an issue when comparing people with different body compositions, those with larger necks artificially generate lower body fat percentage calculations than those with smaller necks. from bmi body fat can be estimated from body mass index (bmi), a person's mass in kilograms divided by the square of the height in meters; if weight is measured in pounds and height in inches, the result can be converted to bmi by multiplying by 703. there are a number of proposed formulae that relate body fat to bmi. these formulae are based on work by researchers published in peer-reviewed journals, but their correlation with body fat are only estimates; body fat cannot be deduced accurately from bmi. body fat may be estimated from the body mass index by formulae derived by deurenberg and co-workers. when making calculations, the relationship between densitometrically determined body fat percentage (bf%) and bmi must take age and sex into account. internal and external cross-validation of the prediction formulas showed that they gave valid estimates of body fat in males and females at all ages. in obese subjects, however, the prediction formulas slightly overestimated the bf%. the prediction error is comparable to the prediction error obtained with other methods of estimating bf%, such as skinfold thickness measurements and bioelectrical impedance. the formula for children is different; the relationship between bmi and bf% in children was found to differ from that in adults due to the height-related increase in bmi in children aged 15 years and younger. child body fat percentage = ( 1.51 × bmi ) − ( 0.70 × age ) − ( 3.6 × sex ) + 1.4 =(1.51\times )-(0.70\times )-(3.6\times )+1.4} adult body fat percentage = ( 1.20 × bmi ) + ( 0.23 × age ) − ( 10.8 × sex ) − 5.4 =(1.20\times )+(0.23\times )-(10.8\times )-5.4} where sex is 0 for females and 1 for males. however – contrary to the aforementioned internal and external cross-validation –, these formulae definitely proved unusable at least for adults and are presented here illustratively only. still, the following formula designed for adults proved to be much more accurate at least for adults: (adult) body fat percentage = ( 1.39 × bmi ) + ( 0.16 × age ) − ( 10.34 × gender ) − 9 =(1.39\times )+(0.16\times )-(10.34\times )-9} where, again, gender (sex) is 0 if female and 1 if male to account for the lower body fat percentage of men. other indices may be used; the body adiposity index was said by its developers to give a direct estimate of body fat percentage, but statistical studies found this not to be so.