Skinfold Body Fat Calculator (Jackson Pollock 3 Site)

Jackson-Pollock 3-Site Skinfold Method: How Body Fat Is Calculated

The skinfold body fat calculator uses the Jackson-Pollock 3-site method, one of the most rigorously validated field protocols for estimating body composition in applied exercise science. Developed by Andrew Jackson and Michael Pollock and first published in 1978, the equations revolutionized field-based body composition assessment by providing a practical, non-invasive alternative to laboratory methods. The method uses subcutaneous fat thickness measured at three specific anatomical landmarks to predict whole-body density (BD), which is then converted to a body fat percentage using the Siri two-compartment model. The sex-specific nature of the equations reflects fundamental physiological differences in fat distribution patterns between men and women.

The Body Density Equations

Because fat distribution patterns differ significantly between biological sexes, separate regression equations apply to men and women:

• Men: BD = 1.10938 - 0.0008267 x S + 0.0000016 x S^2 - 0.0002574 x A
• Women: BD = 1.0994921 - 0.0009929 x S + 0.0000023 x S^2 - 0.0001392 x A

Where S is the sum of the three skinfold measurements in millimeters and A is age in years. The quadratic term (S squared) captures the nonlinear relationship between subcutaneous and total body fat. The age coefficient reflects the well-documented physiological shift toward internal fat storage as adults age, even when surface skinfold readings remain relatively stable over time.

Converting Body Density to Body Fat Percentage

Once body density is determined, the Siri (1956) equation converts it directly to a body fat percentage:

%BF = (495 / BD) - 450

This formula assumes two body compartments with fixed densities: fat mass (approximately 0.9007 g/mL) and fat-free mass (approximately 1.1000 g/mL). The constants 495 and 450 derive mathematically from these assumed densities and are standard across the exercise science literature.

Skinfold Measurement Site Locations

Men: Chest, Abdomen, Thigh

• Chest: A diagonal fold taken halfway between the anterior axillary line and the nipple.
• Abdomen: A vertical fold taken exactly 2 cm to the right of the umbilicus (navel).
• Thigh: A vertical fold on the anterior (front) surface of the thigh, midway between the inguinal crease and the superior border of the patella (kneecap).

Women: Triceps, Suprailiac, Thigh

• Triceps: A vertical fold on the posterior upper arm, midway between the acromion process (shoulder tip) and the olecranon process (elbow point).
• Suprailiac: A diagonal fold taken just above the iliac crest, following the natural diagonal crease of the skin.
• Thigh: Identical location to the male thigh measurement - a vertical fold at the anterior midthigh.

Best Practices for Accurate Measurements

Proper technique is essential for obtaining valid Jackson-Pollock results. Use calibrated skinfold calipers with constant jaw pressure, take all measurements on the right side of the body, and grasp each fold between thumb and forefinger while lifting away from the underlying muscle. Place the caliper jaw perpendicular to the fold at the designated anatomical landmark. For greatest reliability, take duplicate measurements at each site and record the average. Standardized testing conditions—measured at the same time of day and with consistent hydration, meal timing, and exercise timing—enhance accuracy and reduce the impact of temporary fluctuations in subcutaneous water content across repeated assessments.

Worked Calculation Examples

Example 1 - Male, age 30: Chest = 14 mm, Abdomen = 22 mm, Thigh = 14 mm. Sum S = 50 mm.

BD = 1.10938 - 0.0008267(50) + 0.0000016(2500) - 0.0002574(30) = 1.10938 - 0.04134 + 0.00400 - 0.00772 = 1.06432 g/mL

%BF = (495 / 1.06432) - 450 = 465.08 - 450 = 15.1% body fat

Example 2 - Female, age 35: Triceps = 18 mm, Suprailiac = 16 mm, Thigh = 20 mm. Sum S = 54 mm.

BD = 1.0994921 - 0.0009929(54) + 0.0000023(2916) - 0.0001392(35) = 1.0994921 - 0.05362 + 0.00671 - 0.00487 = 1.04771 g/mL

%BF = (495 / 1.04771) - 450 = 472.44 - 450 = 22.4% body fat

Accuracy and Scientific Validation

Peer-reviewed research published by PMC on the validation of skinfold equations and alternative body fat methods confirms that the Jackson-Pollock equations carry a standard error of estimate (SEE) of approximately 3.4-3.7% body fat when compared against hydrostatic (underwater) weighing in adults aged 18-61. Laboratory materials from the University of Michigan Exercise Physiology program document this method as a standard field assessment protocol in applied kinesiology curricula. Accuracy depends on correct anatomical landmark identification, calibrated calipers, and reading the caliper gauge within 2 seconds of applying the pinch. The method is less reliable for individuals with a BMI above 35 or for highly muscular athletes whose subcutaneous-to-total fat ratios may deviate from population norms used to derive the equations.