Body Fat % (Jackson-Pollock 7-Site)

About Body Fat % (Jackson-Pollock 7-Site)

The 7-site Jackson-Pollock equation is the most comprehensive of the generalized skinfold equations developed by Jackson and Pollock in 1978. It samples subcutaneous fat at seven anatomical sites: chest, midaxillary, triceps, subscapular, abdomen, suprailiac, and thigh. By capturing fat from the trunk, the extremities, and both anterior and posterior surfaces, the 7-site equation provides the most representative estimate of total subcutaneous adiposity among the Jackson-Pollock family and is often considered the field reference standard when only skinfold calipers are available.

How the formula works

The 7-site equations output body density, which is converted to percent body fat using Siri's equation (percent fat = 495/density - 450). For men, density = 1.112 - 0.00043499 x sum + 0.00000055 x sum^2 - 0.00028826 x age. For women, density = 1.097 - 0.00046971 x sum + 0.00000056 x sum^2 - 0.00012828 x age. The sum is the total of the seven skinfolds in millimeters. The quadratic term in sum allows the relationship to flatten at higher levels of adiposity, and the age term accounts for the redistribution of fat from subcutaneous to internal depots and the declining density of fat-free mass that occur with aging.

The seven sites

The chest site is a diagonal fold halfway between the anterior axillary line and the nipple. The midaxillary site is a vertical fold at the level of the xiphoid process on the midaxillary line. The triceps is a vertical fold on the posterior mid-upper arm. The subscapular is a diagonal fold 1 to 2 cm below the inferior angle of the scapula. The abdomen is a vertical fold 2 cm lateral to the umbilicus. The suprailiac is a diagonal fold just above the iliac crest in the midaxillary line. The thigh is a vertical fold on the anterior mid-thigh. All measurements should be taken on the right side with calibrated calipers by a trained technician, with each site measured in duplicate or triplicate.

Evidence and validation

The 7-site equation has the largest validation literature of any skinfold method. The original Jackson-Pollock validation reported standard errors of estimate of about 2.7 percent body fat in men and 3.1 percent in women against hydrostatic weighing. Subsequent cross-validation studies across athletes, the elderly, and various ethnic groups have generally confirmed standard errors in the 2.5 to 3.5 percent range, with the 7-site version typically performing as well as or slightly better than the 3- and 4-site versions. The National Health and Nutrition Examination Survey (NHANES) anthropometry procedures manual recommends a similar multi-site skinfold protocol for field use.

Interpretation

Percent body fat values can be interpreted using the American Council on Exercise categories. For men: essential 2-5 percent, athletic 6-13 percent, fitness 14-17 percent, acceptable 18-24 percent, obese 25 percent or more. For women: essential 10-13 percent, athletic 14-20 percent, fitness 21-24 percent, acceptable 25-31 percent, obese 32 percent or more. Population-based reference values from NHANES DEXA data provide more granular age- and sex-specific percentiles.

Limitations

The 7-site method is the most time-consuming of the Jackson-Pollock equations and requires a skilled technician to locate all seven anatomical landmarks consistently. As with all skinfold methods, it assumes a two-component model with fixed densities of fat and fat-free mass, which introduces systematic error in athletes (denser fat-free mass), older adults (less dense fat-free mass), and children. The Siri conversion itself carries an inherent biological variability of about 2 to 3 percent body fat even with perfect skinfold technique, because individual fat-free mass density varies. For research-grade accuracy, four-component models that independently measure body water, mineral, and protein are required.