Body Fat % (Jackson-Pollock 4-Site)

About Body Fat % (Jackson-Pollock 4-Site)

The 4-site Jackson-Pollock-Ward equation extends the landmark Jackson and Pollock 1978 generalized skinfold approach to four measurement sites. Published in Jackson, Pollock, and Ward's 1980 work on generalized equations for predicting body density, the 4-site version uses the abdomen, thigh, triceps, and suprailiac sites for both sexes. By adding a fourth site the equation captures a broader sampling of subcutaneous fat distribution than the 3-site version, often improving reliability in individuals whose fat distribution is atypical, while still using sites that are reasonably easy to locate and measure.

How the formula works

Unlike the 3- and 7-site equations, which output body density that must be converted to percent fat via Siri, the commonly cited 4-site Jackson-Pollock-Ward equations output percent body fat directly. For men the equation is percent fat = 0.29288 x sum - 0.0005 x sum^2 + 0.15845 x age - 5.76377. For women the equation is percent fat = 0.29669 x sum - 0.00043 x sum^2 + 0.02963 x age + 1.4072. The sum is the total of the four skinfolds in millimeters. The linear coefficient on sum captures the dominant effect of subcutaneous fatness, the quadratic term accounts for the non-linear compression of skinfolds at higher adiposity, and the age coefficient reflects the well-documented increase in internal fat and decrease in fat-free mass density with age.

Sites and technique

The four sites are abdomen (vertical fold 2 cm lateral to the umbilicus), thigh (vertical fold on the anterior mid-thigh), triceps (vertical fold on the posterior mid-upper arm), and suprailiac (diagonal fold just above the iliac crest in the midaxillary line). Measurements should be taken on the right side of the body by a trained technician using calibrated calipers, with each site measured in duplicate or triplicate and averaged. The abdomen and suprailiac sites are particularly prone to measurement error if the fold is taken too superficially or too deep.

Evidence and validation

The 4-site equation has been cross-validated in numerous studies against hydrostatic weighing, DEXA, and air displacement plethysmography, with reported standard errors of estimate of about 3.0 to 3.8 percent body fat, comparable to the 3-site and 7-site versions. The addition of the triceps site in men, who normally carry little triceps fat, increases sensitivity to upper-body subcutaneous fat distribution, while in women the four sites together capture the gynoid and android depots reasonably well.

Interpretation

Percent body fat values can be interpreted using standard categories: athletic 6-13 percent (men) and 14-20 percent (women), fitness 14-17 percent and 21-24 percent, acceptable 18-24 percent and 25-31 percent, and obesity above those ranges. Because the 4-site equation outputs percent fat directly, no Siri conversion is required, which avoids one source of propagated error.

Limitations

All skinfold equations share the limitation of operator dependence, with inter-rater variability of 2 to 4 percent body fat even among trained measurers. The 4-site equation, like other Jackson-Pollock generalized equations, was derived in adults and may be less accurate in adolescents, the elderly, and extremely lean or obese individuals. The direct-output form assumes the same two-component model underlying Siri but folds the density conversion into the regression coefficients, so the same biological assumptions about fat and fat-free mass density apply. For research-grade accuracy, four-component models or DEXA are preferred.