How to Calculate Body Fat with Jackson-Pollock 3-Site Skinfolds

 

A skinfold body-fat calculation estimates body density from measured folds of skin and subcutaneous tissue. A second equation converts that estimated density to a body-fat percentage. The result depends on the measurement protocol and population assumptions; it is not a direct measurement of an individual's body composition.

The Body Fat Calculator uses Jackson-Pollock three-site equations and the Siri conversion. It requires skinfold caliper measurements in millimeters, not body weight, tape circumferences or guessed tissue thickness.

Choose the correct three sites

The published men's three-site equation uses chest, abdomen and thigh. The women's equation uses triceps, suprailiac and thigh. These sets are distinct. Measurements from another protocol cannot simply be entered under whichever labels are available.

Use the relevant standardized skinfold procedure, ideally with a trained assessor. Site location, fold direction, caliper placement and repeated-measurement technique affect the values. This calculator does not teach or verify an assessor's technique.

Calculate the sum and body density

Let S be the sum of the three folds in millimeters and A age in years. For the men's equation, density = 1.10938 - 0.0008267 x S + 0.0000016 x S squared - 0.0002574 x A.

For the women's equation, density = 1.0994921 - 0.0009929 x S + 0.0000023 x S squared - 0.0001392 x A. Both are regression equations from published study groups. The calculated density is expressed in grams per cubic centimeter.

Apply the Siri conversion

The calculator then uses body fat (%) = 495 / density - 450. This two-component model makes assumptions about fat and fat-free tissue. Those assumptions do not apply equally to all individuals or populations.

A precise-looking percentage does not remove uncertainty in the folds, density equation or conversion. The difference between two small displayed values may reflect measurement variability rather than a true change in composition.

Work through the example

Choose Published equation group as Men: chest, abdomen, thigh. Enter Age (years) 30, Chest skinfold (mm) 12, Abdomen skinfold (mm) 20 and Thigh skinfold (mm) 16. Select Estimate body fat.

The sum is 12 + 20 + 16 = 48 mm. The men's density equation gives 1.0656628 g/cm3. Applying Siri gives approximately 14.50% body fat, rounded. This example demonstrates arithmetic, not a recommended percentage or a clinical conclusion.

Respect the equation scope

The tool limits the men's group to ages 18-61 and the women's group to ages 18-55, reflecting the age ranges in the original adult studies. It does not extrapolate these equations to children or older adults.

The labels reflect the original published equation groups. Choosing a group does not establish that its calibration is appropriate for you. The calculator does not assess applicability during pregnancy, hormone treatment, clinical conditions or other circumstances that may affect body composition.

Check the units and measurements

Enter every fold in millimeters. A centimeter value entered as millimeters would change the result substantially. The software accepts 2-60 mm per fold as a guardrail, but acceptance does not validate the measurement or the method for that person.

Changing the equation group clears existing folds because the required sites change. Retain measurements in your own records if needed, but do not reuse values from the wrong sites. The tool does not save or transmit them.

Interpret the estimate carefully

No ideal percentages, fitness categories or weight-loss targets are assigned. For a different height-and-weight measure, the Adult BMI Calculator explains BMI; that measure does not directly determine body fat either.

Sources are Jackson and Pollock (1978), Generalized equations for predicting body density of men; Jackson, Pollock and Ward (1980), Generalized equations for predicting body density of women; and Siri (1961), Body composition from fluid spaces and density: analysis of methods.