By Zencalcy Editorial TeamLast reviewed About us
Skinfold measurements estimate body fat by measuring the thickness of a pinch of skin and subcutaneous fat at defined sites. The Jackson-Pollock equations convert the sum of those measurements into body density, and the Siri equation converts density into body fat percentage. Technique matters as much as the math.
Estimate body fat from Jackson-Pollock skinfold sums.
3-site men (chest, abdomen, thigh): density = 1.10938 − 0.0008267S + 0.0000016S² − 0.0002574A 3-site women (triceps, suprailiac, thigh): density = 1.0994921 − 0.0009929S + 0.0000023S² − 0.0001392A 7-site: sex-specific Jackson-Pollock equations using chest, midaxillary, triceps, subscapular, abdomen, suprailiac and thigh Body fat % = (495 ÷ density) − 450
S is the sum of skinfolds in millimetres and A is age in years. The 3-site equations are simpler; the 7-site equation is more robust when measurements are accurate. Results depend heavily on tester skill.
| Symbol | Meaning |
|---|---|
| S | Sum of the selected skinfold sites in millimetres. |
| A | Age in years. |
| Density | Estimated body density in g/mL. |
Density = 1.10938 − 0.0008267 × 45 + 0.0000016 × 45² − 0.0002574 × 30 = 1.10938 − 0.03720 + 0.00324 − 0.00772 = 1.0677. Body fat = 495 ÷ 1.0677 − 450 = 13.6%.
The equations are precise, but the input is not. A skinfold is a pinch of skin and fat, and the boundary between fat and muscle is easy to misjudge. Two testers can differ by several millimetres at the same site, which translates into a meaningful difference in body-fat percentage. The 7-site method is more resistant to a single bad measurement because it averages more sites, but it takes longer and requires more practice. The Siri equation adds another assumption: that the density of fat-free mass is constant, which is not exactly true across ages and ethnicities. None of this makes skinfolds useless. They are cheap, portable and good for tracking change when the same person measures the same sites the same way. If you want a precise absolute number, use DEXA; if you want a practical trend, a consistent caliper protocol works well.
Continue with the ABSI Calculator (A Body Shape Index), Adjusted Body Weight Calculator, or the BAI Calculator (Body Adiposity Index) — or browse more body measurements tools and all health calculators.
Also see the Body Fat Calculator.
A skilled tester can get within about 3–5 percentage points of DEXA. Inexperienced technique can produce much larger errors. Use the same tester and sites for repeat measurements.
Every result is a screening estimate from published anthropometric equations, not a diagnosis or a measurement of body composition. Tape measurements should be taken snugly against the skin without compressing it, at the landmarks named in each tool. Equations were derived in specific populations and can be less accurate for athletes, older adults, children, pregnant people and people with significant oedema.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
Content updated: September 2026