Skinfold Body Fat Calculator (Jackson-Pollock 3-site)

Understanding the skinfold body fat calculator — concept and principle

Skinfold measurement estimates body fat by pinching a double layer of skin and the fat beneath it with a caliper at standard sites. The Jackson-Pollock 3-site method is one of the most widely used protocols because it needs only three sites. This calculator turns your three skinfolds and age into body density and then into body fat percentage using the Siri equation.

Three skinfolds add to 47 mm, become a body density, and the Siri equation turns that into 14.77% fat.

How it is calculated

S = sum of the three skinfolds in millimetres.

Men (chest, abdomen, thigh): density = 1.10938 − 0.0008267 × S + 0.0000016 × S² − 0.0002574 × age
Women (triceps, suprailiac, thigh): density = 1.0994921 − 0.0009929 × S + 0.0000023 × S² − 0.0001392 × age
Siri equation: %BF = 495 ÷ density − 450

Density is in g/cm³; age is in years.

The principle

A large share of body fat lies just under the skin, so thicker skinfolds mean more total fat. Jackson and Pollock measured hundreds of adults by underwater weighing and fitted equations that predict body density from skinfolds and age. The S² term makes the relationship curved. The age term reflects that, at the same skinfolds, older adults carry relatively more internal fat. The Siri equation then converts density into fat percentage by treating the body as two parts, fat at about 0.9 g/cm³ and fat-free tissue at about 1.1 g/cm³.

Worked example

Inputs: male, age 35, chest 12 mm, abdomen 20 mm, thigh 15 mm.
S = 12 + 20 + 15 = 47 mm.
Density = 1.10938 − 0.0008267 × 47 + 0.0000016 × 47² − 0.0002574 × 35 = 1.10938 − 0.03885 + 0.00353 − 0.00901 = 1.06505, shown as 1.07.
%BF = 495 ÷ 1.06505 − 450 = 14.77%.

Each point is the calculator re-run with only “Abdomen (men) / Suprailiac (women) mm” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Abdomen (men) / Suprailiac (women) mm	Body fat (Siri)
5	10.23%
10	11.77%
15	13.28%
20	14.77%
25	16.23%
30	17.67%
40	20.46%

How to read the result

Where does your result fall?

For men, 14.77% falls in the ACE "fitness" range (14–17%); average is 18–24%. For women, fitness is 21–24% and average 25–31%. With a skilled tester the error is typically about ±3–4 percentage points, but an untrained tester can add much more. It is best for tracking change: a falling skinfold sum at steady weight means fat loss.

Common mistakes

Using the wrong sites for the sex: men use chest, abdomen and thigh; women use triceps, suprailiac and thigh.
Measuring each site once. Take at least two readings per site and average them, repeating if they differ by more than 1–2 mm.

Keep in mind: Skinfold results are estimates, not a diagnosis. Accuracy falls for very lean or very obese people, and the equations were developed mainly in white North American adults. Ask a health professional to interpret your results.

Frequently asked questions

Which side of the body should I measure?

Many protocols, including ACSM's, use the right side. What matters most is to use the same side each time.

Why does my body fat change with age even if my skinfolds stay the same?

The age term lowers predicted density, reflecting more internal fat with age, so the same skinfolds give a higher fat estimate.

Sources: Jackson AS, Pollock ML, Generalized equations for predicting body density of men, British Journal of Nutrition 40 (1978) 497–504 · Jackson AS, Pollock ML, Ward A, Generalized equations for predicting body density of women, Medicine and Science in Sports and Exercise 12 (1980) 175–181 · American College of Sports Medicine, ACSM's Guidelines for Exercise Testing and Prescription

Checked 8 Nov 2026. Jackson & Pollock (1978, 1980); Siri (1961).

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