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Skinfold Body Fat (3-Site) Calculator

Enter your sex, age, and three skinfold measurements (men: chest/abdomen/thigh, women: triceps/suprailiac/thigh).

How it is calculated

Enter the three skinfold thicknesses in millimetres and your age in years. The three boxes are labelled only 1st, 2nd and 3rd site, so use the order of the sites for your sex: men chest, abdomen, thigh; women triceps, suprailiac, thigh. With S = the sum of the three, body density is D = 1.10938 − 0.0008267·S + 0.0000016·S² − 0.0002574·age for men and D = 1.0994921 − 0.0009929·S + 0.0000023·S² − 0.0001392·age for women. Body fat is then given by the Siri equation %BF = 495 ÷ D − 450. Any sex other than female uses the male formula. Folds must not be negative and age must be above zero. The result is an estimate, not a medical measurement.

Worked example

A 30-year-old man has chest 10 mm, abdomen 20 mm and thigh 15 mm. The sum is S = 45 mm. Density = 1.10938 − 0.0008267 × 45 + 0.0000016 × 45² − 0.0002574 × 30 = 1.10938 − 0.03720 + 0.00324 − 0.00772 = 1.0677 g/cc. Siri: 495 ÷ 1.0677 − 450 = 463.61 − 450 = 13.61. The tool shows body fat of 13.61%, a sum of 45 mm and a density of 1.0677 g/cc. A woman of the same age with the same three numbers (triceps, suprailiac, thigh) would get 19.06% because her equation differs.

Try this example →

Common mistakes

  • Measuring the wrong sites. The formula assumes chest, abdomen and thigh for men and triceps, suprailiac and thigh for women; entering, say, a biceps fold gives a number that looks valid but is wrong.
  • Entering the fold in centimetres or inches. All three must be in millimetres; 1.5 cm is 15.
  • Entering a single reading rather than the usual practice of repeated measurements. A fold taken once, or taken with a different pinch each time, makes the result jump by several percentage points; take each site again and average.
  • Choosing the wrong sex. The two equations give clearly different answers for identical folds (13.61% against 19.06% here), so the setting matters.
  • Reading the body fat as precise. It is a regression estimate from skin folds, so treat small week-to-week changes with caution and always measure at the same sites and time of day.

Sources

About this calculator

The Jackson-Pollock 3-site skinfold equation estimates body density from three caliper measurements — chest, abdomen and thigh for men; triceps, suprailiac and thigh for women — combined with age in a regression formula, since body fat distribution and skin compressibility both shift with age. That body density is then converted into a body fat percentage using the Siri formula (%BF = (495/density) − 450), which models the body as a two-compartment system of fat mass and fat-free mass with different known densities.

This calculator takes your sex, age and the appropriate three skinfold measurements and returns your estimated body fat percentage using the Jackson-Pollock and Siri equations together. Personal trainers and strength coaches tracking client body composition over time, exercise science students learning field-based body composition methods, and athletes monitoring fat mass changes during a training cycle use it as a low-cost, portable alternative to DEXA or hydrostatic weighing, with the caveat that accuracy depends heavily on consistent, well-trained caliper technique.

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Comments (1)

  • Editorial team

    Caliper error moves this number more than you might expect. On the preset male values, reading each of the three folds just 1 mm too high lifts the result from 13.61% to 14.50%, close to a full percentage point. Mark the sites, use the same pinch pressure and take the average of repeated readings, then compare like with like over time rather than chasing a single figure. Which of the three sites do you find hardest to measure consistently?

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