Body Fat % (Jackson-Pollock 3-Site)

About Body Fat % (Jackson-Pollock 3-Site)

The Jackson-Pollock generalized skinfold equations, published by A. S. Jackson and M. L. Pollock in 1978 in the British Journal of Nutrition, are among the most validated and widely used field methods for estimating body composition. The authors analyzed skinfold measurements at multiple sites in several hundred men and women across a wide age range and developed generalized (non-population-specific) polynomial equations that predict body density from the sum of skinfolds and age. The resulting body density is then converted to percent body fat using the Siri (1961) equation, percent fat = 495/density - 450, which assumes the density of fat is 0.900 g/cc and of the fat-free mass is 1.100 g/cc.

Sites and how the formula works

The 3-site equations use sex-specific sites. For men the sites are chest (a diagonal fold halfway between the anterior axillary line and the nipple), abdomen (a vertical fold 2 cm to the right of the umbilicus), and thigh (a vertical fold on the front of the thigh midway between the hip and knee). For women the sites are triceps (a vertical fold on the back of the upper arm midway between the shoulder and elbow), suprailiac (a diagonal fold just above the iliac crest in the midaxillary line), and thigh (as in men). The sum of the three skinfolds (in millimeters) and the person's age are entered into sex-specific equations that yield body density. The male equation is density = 1.10938 - 0.0008267 x sum + 0.0000016 x sum^2 - 0.0002574 x age. The female equation is density = 1.0994921 - 0.0009929 x sum + 0.0000023 x sum^2 - 0.0001392 x age. The quadratic term in sum reflects the diminishing increase in density per unit skinfold at higher levels of adiposity.

Measurement technique

Skinfold measurements require calibrated calipers (Lange, Harpenden, or Slim Guide) and trained technicians. Each fold is grasped firmly between thumb and index finger, lifted away from the underlying muscle, and measured with the caliper about 1 cm below the fingers. Measurements are taken on the right side of the body, and duplicate or triplicate readings are averaged; if two readings differ by more than 1 to 2 mm, a third is taken. Reliability depends heavily on technician skill, with inter-rater correlations typically above 0.9 for experienced measurers but considerably lower for novices.

Evidence and validation

The original Jackson-Pollock validation reported standard errors of estimate of about 3.3 percent body fat in men and 3.6 percent in women against hydrostatic weighing. A large subsequent literature has broadly confirmed these figures across diverse populations, including athletes, older adults, and various ethnic groups, although some studies find systematic underestimation in obese individuals and overestimation in very lean athletes. The 3-site equations are the most commonly used in fitness and corporate wellness settings because they balance accuracy and measurement burden.

Interpretation

Percent body fat values from the Jackson-Pollock equations can be interpreted using the American Council on Exercise categories or the more granular ranges derived from NHANES DEXA data. For men, athletic is typically 6-13 percent, fitness 14-17 percent, acceptable 18-24 percent, and obese 25 percent or more. For women, athletic 14-20 percent, fitness 21-24 percent, acceptable 25-31 percent, and obese 32 percent or more. Essential fat is 2-5 percent in men and 10-13 percent in women.

Limitations

The Siri two-component model assumes constant densities of fat and fat-free mass, which breaks down in athletes (denser fat-free mass due to high bone and muscle mineral), older adults (less dense fat-free mass due to bone loss), and children. The equations were derived in adults aged 18 to 61 and should be applied cautiously outside that range. Skinfold technique is highly operator-dependent, and individual biological variability in fat-free mass density adds irreducible error of 2 to 3 percent body fat even with perfect technique.