How the skinfold body fat calculator works
This tool implements the Jackson-Pollock 3-site skinfold method, one of the most widely used field techniques for estimating body fat percentage. A caliper measures the thickness of a pinched fold of skin plus underlying fat at three standard sites, and a regression equation converts the sum of those folds - along with age - into an estimate of whole-body density. The Siri equation then converts body density into body fat percentage.
How the math works
- Measurement sites: for men, the three sites are chest, abdomen, and thigh. For women, the three sites are triceps, suprailiac, and thigh. Each fold is measured in millimeters, ideally as the average of two or three readings per site with a calibrated skinfold caliper.
- Body density (Jackson-Pollock, 1978): men: Db = 1.10938 - 0.0008267 x sum3 + 0.0000016 x sum3^2 - 0.0002574 x age. Women: Db = 1.0994921 - 0.0009929 x sum3 + 0.0000023 x sum3^2 - 0.0001392 x age, where sum3 is the sum of the three skinfolds in mm.
- Body fat percentage (Siri, 1961): body fat % = (495 / Db) - 450. Fat mass is body weight x body fat %, and lean mass is body weight minus fat mass.
Common measurement mistakes
- Measuring the wrong sites for the person's sex, or mixing site conventions from a different protocol (such as a 4- or 7-site method).
- Taking a single reading per site instead of averaging two or three - skinfold thickness is easy to misjudge on any one pinch.
- Comparing skinfold results across different testers. Because caliper technique varies by person, the most reliable trend data comes from the same tester using the same sites each time.
Frequently Asked Questions
Which skinfold sites does this calculator use?
It uses the Jackson-Pollock 3-site protocol. For men the three sites are chest, abdomen, and thigh. For women the three sites are triceps, suprailiac, and thigh. Enter each fold in millimeters as measured with a skinfold caliper, ideally as the average of two or three readings per site.
How is body fat percentage calculated from skinfolds?
The three fold thicknesses are summed, then plugged into a sex-specific Jackson-Pollock regression along with age to estimate body density. The Siri equation then converts body density to body fat percentage: body fat % = (495 / body density) - 450. Fat mass and lean mass follow from body fat % and body weight.
Is this result a medical diagnosis?
No - this calculator provides a population-based estimate, not a diagnosis. Skinfold accuracy depends heavily on caliper technique and site location, and typical error versus lab methods like DEXA is a few percentage points. Use it to track trends, and consult a professional for clinical assessment.