About the Skinfold Body Fat
The Skinfold Body Fat Calculator estimates body fat percentage from three caliper measurements using the Jackson-Pollock 3-site protocol, one of the most widely validated skinfold methods. It's built for anyone who owns a set of skinfold calipers and wants a body composition estimate without access to a DEXA scan or hydrostatic weighing. Because the formula and measurement sites differ by sex, the calculator uses separate equations and site definitions for men and women.
How It Works
You enter your sex, age, and three skinfold thickness readings in millimeters: for men, chest, abdomen, and thigh; for women, tricep, suprailiac, and thigh. The calculator sums the three readings and plugs that sum, along with your age, into a sex-specific regression equation to estimate body density, then converts that density into a body fat percentage using the Siri equation. The result is floored at zero so an unusual input combination can't return a negative percentage.
Formula & Methodology
To calculate by hand, first add your three site readings to get S. Square S for the quadratic term and multiply it by the small positive coefficient (0.0000016 for men, 0.0000023 for women), then subtract the linear term (S multiplied by 0.0008267 for men or 0.0009929 for women) and the age term (age multiplied by 0.0002574 for men or 0.0001392 for women) from the equation's base constant. That gives body density, typically a number just above 1.0. Dividing 495 by that density and subtracting 450 converts it into an estimated body fat percentage.
Examples
Male estimate at default inputs
A 30-year-old man measures 12mm at the chest, 15mm at the abdomen, and 18mm at the thigh, for a sum of 45mm. Running the male equation gives a body density of about 1.0677, which converts to roughly 13.6% body fat.
Female estimate with higher skinfolds
A 40-year-old woman measures 15mm at the tricep, 20mm at the suprailiac, and 22mm at the thigh, for a sum of 57mm. The female equation gives a body density of about 1.0448, which converts to roughly 23.8% body fat.
Advantages
- Uses a specific, published skinfold protocol (Jackson-Pollock 3-site) rather than a generic or unnamed body fat estimate.
- Accounts for both sex and age in the density calculation, rather than treating everyone with the same skinfold sum identically.
- Requires only a caliper and three measurements, making repeat testing to track change over time fast and low-cost.
Common Mistakes
- Measuring the wrong sites for your sex, since the three site locations are entirely different between the male and female protocol.
- Inconsistent caliper technique between sessions, such as pinching a different amount of skin fold or waiting a different number of seconds before reading the caliper, which introduces variability unrelated to actual body fat change.
- Treating a single reading as a precise absolute number rather than an estimate best used to track relative trends across repeated, consistently taken measurements.
Edge Cases to Watch For
- The result is clamped at a minimum of 0%, so extremely low skinfold sums, which would otherwise mathematically produce a negative percentage, still return 0% rather than an invalid figure.
- Men and women measure entirely different anatomical sites (chest, abdomen, thigh versus tricep, suprailiac, thigh), so the labels "Site 1," "Site 2," and "Site 3" mean different body locations depending on the sex selected.
- The regression equations were developed and validated on general adult populations - accuracy tends to degrade at the extremes of body composition, such as very lean athletes or individuals with obesity, compared to the population the formula was built from.
Common Use Cases
- Personal trainers and coaches performing routine body composition checks on clients without access to lab equipment.
- Individuals in a training program who want to track body fat changes over time using a consistent, repeatable method.
- Athletes wanting a sex- and age-adjusted estimate rather than a generic BMI-based body fat approximation.