Free tool · Body

Skinfold body fat calculator (3-site)

Short answer

The Jackson-Pollock three-site method turns three caliper skinfolds and your age into body density, then Siri's equation turns density into body fat. Men measure the chest, abdomen, thigh; women the triceps, suprailiac, thigh. A 30-year-old man whose three skinfolds add up to 60 mm comes out at about 17.9% body fat. Expect an error of about 3 to 4 percentage points.

Last reviewed 27 October 20263 published sourcesNot medical advice

Reps keeps track of this for you on iPhone, iPad and Apple Watch.

Get Reps free

Skinfold body fat calculator

Measure the three sites for your sex with skinfold calipers, the same way each time.

Enter your age and three skinfold measurements.

Runs entirely in your browser. Nothing you type is saved or sent anywhere.

Estimated body fat (%) by sum of three skinfolds and age
Sum of skinfolds	Men 25	Men 40	Men 55	Women 25	Women 40	Women 55
20 mm	5.4	7.0	8.6	9.6	10.5	11.4
40 mm	11.6	13.2	14.9	17.0	17.9	18.8
60 mm	17.4	19.1	20.8	23.8	24.8	25.7
80 mm	22.8	24.5	26.3	30.0	31.0	31.9
100 mm	27.7	29.5	31.3	35.4	36.4	37.4
120 mm	32.1	33.9	35.7	40.1	41.1	42.1

Men: chest, abdomen, thigh. Women: triceps, suprailiac, thigh.

The equations

Men: body density = 1.10938 − 0.0008267 × S + 0.0000016 × S² − 0.0002574 × age, where S is the chest, abdomen and thigh skinfolds added together in millimetres (Jackson & Pollock, 1978).

Women: body density = 1.0994921 − 0.0009929 × S + 0.0000023 × S² − 0.0001392 × age, where S is the triceps, suprailiac and thigh skinfolds (Jackson, Pollock & Ward, 1980).

Siri: body fat % = 495 ÷ body density − 450 (Siri, 1961).

How well it works

Jackson and Pollock built the men's equations on 308 men aged 18 to 61 and checked them on another 95, with standard errors of about 0.0073 to 0.0077 g/mL of body density (Jackson & Pollock, 1978). The women's equations, from 249 women aged 18 to 55, had standard errors of 3.6 to 3.8% fat, and 3.7 to 4.0% when checked on 82 more women (Jackson, Pollock & Ward, 1980).

No calipers? The body fat calculator uses a tape measure instead, and the lean body mass calculator uses height and weight.

Body changes, tracked over time

Reps keeps your weight and body measurements from Apple Health next to your nutrition and training, so a trend is easy to see.

Download Reps on the App Store Nutrition in Reps

Free to download · iPhone, iPad and Apple Watch

Common questions

Which skinfold sites do I measure?

Men use the chest, abdomen, thigh. Women use the triceps, suprailiac, thigh. These are the three-site combinations from the Jackson-Pollock equations for men and women.

How accurate are skinfold calipers?

In the original studies, the equations predicted lab-measured body density with a standard error of about 3 to 4 percentage points of body fat. Measurement technique adds more, so the same person measuring the same way each time gives the most useful trend.

Does age matter?

Yes. Age is part of both equations. The men's equation was built on men aged 18 to 61 and the women's on women aged 18 to 55; the authors of the women's study advise care with women over 40.

Sources

Figures on this page were checked against these publications on 27 October 2026.

Jackson AS, Pollock ML. Generalized equations for predicting body density of men. British Journal of Nutrition. 1978;40(3):497–504. doi.org/10.1079/BJN19780152
Jackson AS, Pollock ML, Ward A. Generalized equations for predicting body density of women. Medicine & Science in Sports & Exercise. 1980;12(3):175–181. doi.org/10.1249/00005768-198023000-00009
Siri WE. Body composition from fluid spaces and density: analysis of methods. 1961. Reprinted in Nutrition. 1993;9(5):480–491. pubmed.ncbi.nlm.nih.gov/8286893/

More body tools

See every free tool →