Three-Site Skinfold CalculatorFree online fitness & health calculator — instant results, no sign-up
Estimate adult fat percentage with Jackson-Pollock three-site equations.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
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About the Three-Site Skinfold Calculator
The Three-Site Skinfold Calculator estimates adult body fat percentage using Jackson–Pollock three-site equations and the Siri conversion. It combines age with chest, abdomen, and thigh skinfolds for the male formula, or triceps, suprailiac, and thigh skinfolds for the female formula. Measurements must be skinfold thicknesses taken with calipers in millimeters, not body circumferences.
- Sex used by formula [options: Male, Female]
- Select Male or Female to choose the corresponding equation and measurement sites; this unitless selection defaults to Male.
- Age (years)
- Enter the adult's age in years; the default is 30.
- Chest (male) / triceps (female) (mm)
- Enter the chest skinfold for the male formula or the triceps skinfold for the female formula in millimeters; the default is 15 mm.
- Abdomen (male) / suprailiac (female) (mm)
- Enter the abdomen skinfold for the male formula or the suprailiac skinfold for the female formula in millimeters; the default is 22 mm.
- Thigh skinfold (mm)
- Enter the thigh skinfold thickness in millimeters for either formula; the default is 20 mm.
Formula and method
S = x₁ + x₂ + x₃. Male: D = 1.10938 − 0.0008267S + 0.0000016S² − 0.0002574A. Female: D = 1.0994921 − 0.0009929S + 0.0000023S² − 0.0001392A. Siri body fat: BF = 495/D − 450. With Male, A = 30, x₁ = 15 mm, x₂ = 22 mm, and x₃ = 20 mm: S = 57 mm; D = 1.05973792 g/cm³; BF rounds to 17.1%.
- AAge in years.
- x₁Chest skinfold for the male formula or triceps skinfold for the female formula, in millimeters.
- x₂Abdomen skinfold for the male formula or suprailiac skinfold for the female formula, in millimeters.
- x₃Thigh skinfold thickness in millimeters.
- SSum of the three skinfold thicknesses in millimeters.
- DEstimated body density in grams per cubic centimeter.
- BFEstimated body fat as a percentage of total body mass.
The calculator sums the three skinfolds, then substitutes that sum and age into the selected Jackson–Pollock equation to estimate body density. It converts density to body fat percentage using the Siri equation and rounds the displayed result to one decimal place.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Sex used by formulaMale
- Age30 years
- Chest (male) / triceps (female)15 mm
- Abdomen (male) / suprailiac (female)22 mm
- Thigh skinfold20 mm
Results
- Estimated body fat (Siri)17.1 %
- NoteEducational estimate for adults; not a diagnosis or treatment recommendation.
How to use the Three-Site Skinfold Calculator
- 1Select the sex used by the formula and enter age in years; the default example uses Male and 30 years.
- 2Identify the correct sites: chest, abdomen, and thigh for Male, or triceps, suprailiac, and thigh for Female.
- 3Use skinfold calipers and a consistent measurement protocol, preferably with a trained measurer; repeat measurements at each site to check consistency.
- 4Enter the site measurements in millimeters; the default male example uses chest 15 mm, abdomen 22 mm, and thigh 20 mm.
- 5Read the estimated body fat and the accompanying note; the default inputs produce 17.1%.
Reading your result
“Estimated body fat (Siri) = 17.1 %” means that the model estimates fat accounts for 17.1% of total body mass with the default inputs. It is not a direct measurement or a health classification. “Note = Educational estimate for adults; not a diagnosis or treatment recommendation.” explains the intended use and applies regardless of the percentage shown.
Limitations
Accuracy depends on correct anatomical landmarks, caliper technique, and repeatable measurements. The original equations were developed in adult samples with limited age ranges, approximately 18–61 years for men and 18–55 years for women; estimates outside those ranges are less well supported. Results may be less accurate for people whose fat distribution or body composition differs substantially from the development samples. The Siri conversion assumes relatively constant densities of fat and fat-free tissue, which do not hold equally for everyone. This tool is not intended for children or pregnancy, and the binary formula choices do not capture all sex-related or hormone-related variation in body composition. Small changes may reflect measurement error rather than actual fat change.
Common questions
Can I select Female while keeping chest and abdomen measurements?+
No. The female equation requires triceps, suprailiac, and thigh measurements. Changing the formula selection also changes the anatomical sites represented by the first two skinfold fields.
Where are the three skinfold sites measured?+
For the standard male protocol, the chest fold is diagonal, approximately halfway between the anterior armpit line and nipple, and the abdomen fold is vertical, about 2 cm to the right of the navel. For the female protocol, the triceps fold is vertical on the back of the upper arm, midway between shoulder and elbow, and the suprailiac fold is diagonal just above the iliac crest at the anterior armpit line. The thigh fold is vertical on the front of the thigh, midway between the inguinal crease and the upper edge of the kneecap. A trained measurer can help locate these landmarks consistently.
Why does age change the result when the three skinfolds stay the same?+
Both Jackson–Pollock equations include an age term. At a fixed skinfold sum, increasing age lowers estimated body density and therefore increases the Siri body fat estimate.
Why do the default measurements produce 17.1% rather than 57%?+
The default measurements sum to 57 mm, which is a thickness total, not a percentage. The male equation combines that sum with age 30 to estimate density, and the Siri conversion turns that density into 17.1% body fat after rounding.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.