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Jackson & Pollock 7-site skinfold protocol: formula and calculator

By PounceFit Team · Updated on

Skinfold caliper and measuring tape on a table
Photo: skamille (CC BY 2.0), via Flickr

Technical content for health and fitness professionals. It reproduces formulas and protocols published in the scientific literature, with the sources at the end of the page. It is not a recommendation and does not replace the judgment of the professional who runs the assessment.

The Jackson & Pollock 7-site skinfold protocol estimates body fat percentage from the sum of seven skinfolds and age: one equation calculates body density, with a version for men and another for women, and the Siri equation converts density to percent body fat (Jackson and Pollock, 1978; Jackson, Pollock and Ward, 1980; Siri, 1961). The seven sites are chest, midaxillary, triceps, subscapular, abdomen, suprailiac and thigh. To run the numbers, use the 7-site skinfold calculator.

It is the default protocol in PounceFit, body assessment software for caliper assessments. To record the measurements on paper, the printable body assessment form has fields for the 7 sites.

The formulas in full

S is the sum of the seven skinfolds, in millimeters. Age is in completed years.

Men (Jackson and Pollock, 1978):

Body density = 1.112 − 0.00043499 × S + 0.00000055 × S² − 0.00028826 × age

Women (Jackson, Pollock and Ward, 1980):

Body density = 1.097 − 0.00046971 × S + 0.00000056 × S² − 0.00012828 × age

Conversion to body fat percentage (Siri, 1961):

Body fat (%) = (4.95 ÷ density − 4.50) × 100

With the percentage and body mass, you get fat mass (mass × percentage ÷ 100) and fat-free mass (mass − fat mass).

Where to measure each skinfold

The table summarizes the sites described by the ACSM (2021). Measure on the right side, with the client standing and relaxed.

Skinfold Direction Site
Chest Diagonal Men: halfway between the anterior axillary line and the nipple. Women: one third of that distance, from the axillary line.
Midaxillary Vertical Midaxillary line, at the level of the xiphoid process of the sternum.
Triceps Vertical Back of the upper arm, midway between the acromion and the olecranon.
Subscapular Diagonal (about 45°) 1 to 2 cm below the inferior angle of the scapula.
Abdomen Vertical 2 cm to the right of the umbilicus.
Suprailiac Diagonal Just above the iliac crest, on the anterior axillary line.
Thigh Vertical Front of the thigh, midway between the inguinal crease and the top of the patella.

Take at least two measurements at each site, in rotation, and use the mean. If the two differ by too much, measure again (ACSM, 2021). PounceFit asks for two measurements per site and averages them for you.

Worked example

Man, 30 years old, 80 kg (176 lb), with these skinfolds (mean of measurements, in mm): chest 10, midaxillary 12, triceps 11, subscapular 15, abdomen 22, suprailiac 16 and thigh 14.

  1. Skinfold sum: S = 10 + 12 + 11 + 15 + 22 + 16 + 14 = 100 mm.
  2. S² = 10,000.
  3. Density = 1.112 − 0.00043499 × 100 + 0.00000055 × 10,000 − 0.00028826 × 30 = 1.112 − 0.043499 + 0.0055 − 0.0086478 = 1.0653532.
  4. Body fat = (4.95 ÷ 1.0653532 − 4.50) × 100 = 14.6%.
  5. Fat mass = 80 × 14.6 ÷ 100 = 11.7 kg. Fat-free mass = 80 − 11.7 = 68.3 kg.

The calculator and the app reach 14.63%, 11.7 kg and 68.3 kg with these same numbers.

Limits of the method

  • Source population. The equations came from general-population adults in the United States, with density measured by hydrostatic weighing: 308 men aged 18 to 61 and 249 women aged 18 to 55 (Jackson and Pollock, 1978; Jackson, Pollock and Ward, 1980). In other populations and age ranges, accuracy may be lower (Heyward and Wagner, 2004).
  • Estimation error. Even when measured well, skinfold equations estimate body fat with an error of about 3 to 4 percentage points against the reference method (Heyward and Wagner, 2004). The result is an estimate, not a direct measurement.
  • Siri conversion. The Siri equation assumes fixed densities for fat mass (0.90 g/cm³) and fat-free mass (1.10 g/cm³) (Siri, 1961). For groups whose fat-free mass departs from this value, such as children and older adults, specific conversion formulas exist (Heyward and Wagner, 2004).
  • Technique. Assessor, caliper and site location weigh heavily on the result. The ISAK standards (Stewart et al., 2011) and assessor training reduce this variation.
  • Tracking. To compare assessments, keep the same protocol, the same assessor and the same caliper. The skinfold sum in millimeters also tracks change without going through the equation (Heyward and Wagner, 2004).

If you are unsure which version to use, see Jackson & Pollock 3-site vs 7-site.

Frequently asked questions

What is the difference between the Jackson & Pollock 7-site and 3-site protocols?

Both versions come from the same studies (Jackson and Pollock, 1978; Jackson, Pollock and Ward, 1980) and use the same approach: body density from the skinfold sum and age, converted to body fat with the Siri equation. The 3-site version is faster and uses different sites for men and women. The 7-site version covers more body regions, which reduces the weight of a single poorly measured fold in the result.

Can I use this protocol with athletes or adolescents?

The equations were developed in general-population adults: men aged 18 to 61 and women aged 18 to 55 (Jackson and Pollock, 1978; Jackson, Pollock and Ward, 1980). Outside these ranges, or in groups with very different body composition, the error tends to be larger, and specific equations exist for some groups (Heyward and Wagner, 2004). Record the protocol you used and keep it for reassessments.

Why do two assessors get different results on the same client?

Because a skinfold depends on the exact site, how the skin is pinched and when the caliper is read. The ISAK standards exist to reduce this difference (Stewart et al., 2011). In practice, the same assessor, with the same caliper and the same technique, is what makes reassessments comparable.

Which side of the body should I measure?

The right side, as recommended by the ACSM (2021) and ISAK (Stewart et al., 2011). What matters most is measuring the same side at every assessment of that client.

Why Siri and not Brozek to convert density?

Both equations convert density to body fat using the two-compartment model and give close values across the usual density range (Heyward and Wagner, 2004). PounceFit uses Siri (1961), the conversion used in the Jackson and Pollock studies.

Sources

  1. Jackson AS, Pollock ML. Generalized equations for predicting body density of men. British Journal of Nutrition. 1978;40(3):497-504. doi:10.1079/BJN19780152
  2. Jackson AS, Pollock ML, Ward A. Generalized equations for predicting body density of women. Medicine and Science in Sports and Exercise. 1980;12(3):175-181.
  3. Siri WE. Body composition from fluid spaces and density: analysis of methods. In: Brožek J, Henschel A, editors. Techniques for measuring body composition. Washington: National Academy of Sciences; 1961. p. 223-244.
  4. Heyward VH, Wagner DR. Applied body composition assessment. 2nd ed. Champaign: Human Kinetics; 2004.
  5. American College of Sports Medicine. ACSM's guidelines for exercise testing and prescription. 11th ed. Philadelphia: Wolters Kluwer; 2021.
  6. Stewart A, Marfell-Jones M, Olds T, de Ridder H. International standards for anthropometric assessment. Lower Hutt: International Society for the Advancement of Kinanthropometry; 2011.