Body Composition from Skinfolds (fat %, fat mass, FFM, FFMI, FMI)

Why use it

BMI cannot distinguish a muscular person from an adipose one, and weight alone cannot tell whether a loss is fat or lean tissue. Skinfolds are the cheapest field method that separates the two compartments, and the indices make the result comparable between people of different heights.

About

Estimates body composition from caliper measurements. In adults the skinfold sum predicts body density, which a two-compartment model converts to fat percentage; in children and adolescents that model does not hold, so the Slaughter equations are used to go from skinfolds to fat percentage directly. From the fat percentage the calculator derives fat mass, fat-free mass, and the fat-free mass index and fat mass index, which normalise those masses for height in the same way BMI normalises weight.

When to use

• Nutritional assessment where fat and lean mass must be distinguished, not just weight
• Following an intervention, to show whether the change was in fat or in lean tissue
• Athletes and physically active people, in whom BMI misclassifies routinely
• Screening for sarcopenia or for low fat-free mass in older adults
• Paediatric adiposity assessment where BMI-for-age alone is ambiguous

Formula

Pearls & pitfalls

• The measurement error is in the calipers, not the arithmetic. Take each site in triplicate on the right side, read two seconds after the jaws are released, and use the median — inter-observer differences dwarf the difference between equations.
• Fat-free mass index is what reveals what BMI hides: two adults at BMI 27 can sit at FFMI 22 (muscular) or 16 with a high fat mass index (adipose), and the management differs completely.
• Jackson & Pollock were derived on relatively lean adults and lose accuracy above about 35% body fat, where skinfolds exceed the caliper jaw range and compress unpredictably.
• Siri and Brožek differ by under one percentage point through the normal range; the choice between them matters far less than the quality of the skinfold measurement.
• In children, Slaughter is preferred precisely because it does not pass through density: the fat-free mass of a child is more hydrated and less mineralised, so the adult assumption of 1.100 g/cm³ overestimates their fat.

Next steps

Fat-free mass feeds directly into the Cunningham equation for resting energy expenditure, so pair this with the energy-expenditure calculator when planning intake. A low fat-free mass index in an older adult should prompt formal sarcopenia assessment including grip strength and gait speed. In children, read the result alongside BMI-for-age rather than instead of it.

Evidence

Durnin & Womersley (Br J Nutr 1974) derived the 4-site log-linear equations on 481 adults aged 16-72 with underwater weighing. Jackson & Pollock published the generalised quadratic equations for men (Br J Nutr 1978) and women (Med Sci Sports Exerc 1980). Siri (1961) and Brožek (1963) are the classical two-compartment density-to-fat conversions. Slaughter et al. (Hum Biol 1988) derived the paediatric equations against a multi-component criterion in 310 children and youths, explicitly to avoid the adult fat-free-mass density assumption.

References

1. Durnin JVGA, Womersley J. Body fat assessed from total body density and its estimation from skinfold thickness: measurements on 481 men and women aged from 16 to 72 years. Br J Nutr. 1974;32(1):77-97. — link
2. Jackson AS, Pollock ML. Generalized equations for predicting body density of men. Br J Nutr. 1978;40(3):497-504. — link
3. Jackson AS, Pollock ML, Ward A. Generalized equations for predicting body density of women. Med Sci Sports Exerc. 1980;12(3):175-181. — link
4. Siri WE. Body composition from fluid spaces and density: analysis of methods. In: Techniques for Measuring Body Composition. Washington DC: National Academy of Sciences; 1961:223-244. — link
5. Slaughter MH, Lohman TG, Boileau RA, et al. Skinfold equations for estimation of body fatness in children and youth. Hum Biol. 1988;60(5):709-723. — link
6. VanItallie TB, Yang MU, Heymsfield SB, Funk RC, Boileau RA. Height-normalized indices of the body's fat-free mass and fat mass. Am J Clin Nutr. 1990;52(6):953-959. — link