Key Takeaways
- The Jackson-Pollock 7-site method uses sex- and age-specific regression equations to predict body density from seven skinfold measurements.[1]
- Body density is converted to body fat percentage using either the Siri equation (BF% = 495/BD − 450) or the Brozek equation (BF% = 457/BD − 414.2).[1]
- The 7-site model provides higher accuracy than the 3-site version by capturing more body regions, though it requires greater measurement skill and time.[3]
- Both males and females use the same seven skinfold sites but different regression coefficients, reflecting sex-specific fat distribution patterns.[2]
Formula Explanation
The original equations were published by Jackson and Pollock (1978) for males and Jackson, Pollock, and Ward (1980) for females. The formulas use a quadratic polynomial:
Males: BD = 1.112 − 0.00043499(S) + 0.00000055(S²) − 0.00028826(age)
Females: BD = 1.097 − 0.00046971(S) + 0.00000056(S²) − 0.00012828(age)
S = sum of 7 skinfolds in mm; BD = body density in g/cm³
The quadratic term (S²) accounts for the non-linear relationship between skinfold thickness and body density. Age is included because body composition changes with age even at the same skinfold thickness.
Comparison with 3-Site Method
The Jackson-Pollock 3-Site method uses three sex-specific sites and is faster to administer. The 7-site method adds four more sites (midaxillary, triceps, subscapular, and abdominal for males; chest and midaxillary for females), providing a more comprehensive assessment at the cost of more measurement time.
For a 4-site approach using the same sites for both sexes, see the Jackson-Pollock 4-Site. For an alternative 4-site method with age-band coefficients, see the Durnin-Womersley 4-Site. For the most comprehensive skinfold assessment with nine sites, check the Parrillo 9-Site.
Non-skinfold options include the Navy Body Fat Calculator (circumference-based) and the Relative Fat Mass (height and waist-based).
Accuracy and Limitations
- The JP 7-site equations were developed and validated on adult populations. Results may be less accurate for adolescents or older adults outside the validation sample.[4]
- Skinfold measurements require a trained assessor. Inter-tester variability is the largest source of error.
- The Siri and Brozek equations assume constant densities for fat and fat-free mass, which may not hold for all populations.
- Very lean or very obese individuals may fall outside the accurate range of the regression equation.
- This calculator provides informational estimates for body composition awareness and personal tracking.
Frequently Asked Questions
Sources & References
- Generalized Equations for Predicting Body Density of Men. Jackson AS, Pollock ML. British Journal of Nutrition (1978)
- Generalized Equations for Predicting Body Density of Women. Jackson AS, Pollock ML, Ward A. Medicine & Science in Sports & Exercise (1980)
- Comparison of 7-Site Skinfold Measurement and Dual-Energy X-ray Absorptiometry for Estimating Body Fat Percentage. Kuo FC, Lu CH, Wu LW, Kao TW, Su SC, Liu JS, Chen KC, Chang CH, Kuo CC, Lee CH, Hsieh CH. PLoS One (2020)
- Validity of Skinfold-Based Body Fat Equations for Diverse Populations. Various. Clinical Physiology and Functional Imaging (2018)
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