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What the Jackson-Pollock 3-Site Method Actually Measures
Skinfold testing works on a simple physiological idea: roughly half of your total body fat sits directly under the skin, so pinching a fold of skin plus its underlying fat layer at specific sites gives a proxy for how much fat you're carrying overall. This calculator uses the Jackson-Pollock 3-site protocol because it's the most widely used skinfold method in gyms and research settings — it only requires three measurement sites instead of seven, which makes it faster to administer without sacrificing much accuracy compared to the longer protocols. Unlike the U.S. Navy tape-measure method, this one requires a purpose-built tool: a skinfold caliper, which applies a fixed, calibrated pinch pressure so that different testers (in theory) get comparable readings.
The Formula Behind the Estimate
Jackson and Pollock developed their generalized equations in 1978 from a large sample of adults, using hydrostatic (underwater) weighing as the accuracy benchmark. The 3-site protocol differs by gender because men and women tend to accumulate subcutaneous fat in different regions. For men, you measure the chest, abdomen, and thigh, sum the three readings in millimeters (S), and apply: Body Density = 1.10938 − 0.0008267×S + 0.0000016×S² − 0.0002574×age. For women, you measure triceps, suprailiac, and thigh, and apply: Body Density = 1.0994921 − 0.0009929×S + 0.0000023×S² − 0.0001392×age. Both equations include age because skin compressibility and fat distribution change over the lifespan, and both include a squared term because the relationship between skinfold thickness and density isn't perfectly linear — it flattens out at higher fat levels. Once you have body density, the Siri equation converts it into a percentage: Body Fat % = (495 ÷ Body Density) − 450. This calculator runs both steps for you and shows the intermediate density figure alongside your final percentage.
How to Take Accurate Skinfold Measurements
Your result is only as reliable as your caliper technique, which is the single biggest source of error in this method. For each site, firmly pinch a fold of skin and the fat directly beneath it — but not the underlying muscle — between your thumb and forefinger, then place the caliper jaws about a centimeter below your fingers and let them close fully. Read the dial roughly one to two seconds after the jaws settle, since skinfolds compress slightly under sustained pressure. The chest site is a diagonal fold halfway between the nipple and the armpit crease; the abdomen is a vertical fold about 2cm to the side of the navel; the thigh is a vertical fold on the front of the thigh, midway between the hip and the kneecap. For women, the triceps site is a vertical fold on the back of the upper arm, midway between the shoulder and elbow, and the suprailiac is a diagonal fold just above the hip bone. Take each measurement two or three times and use the average, and where possible have the same person measure you every time — technique varies enough between testers that switching testers can shift your number more than an actual change in body fat would.
Using Your Result and Its Limits
Once calculated, your body fat percentage is placed into a standard category range so you have context for where it falls. I'd treat any single reading as a snapshot rather than a precise verdict — the real value of skinfold testing comes from repeated measurements under consistent conditions (same tester, same sites, similar time of day) so you can track genuine changes in body composition over weeks and months. Keep in mind that this method requires a skinfold caliper — a tape measure or bathroom scale cannot substitute for it — and that accuracy drops noticeably with an inexperienced tester or inconsistent technique. It also tends to be less reliable at the extremes of leanness and body size, since the underlying regression was built from a general adult population. If you need a highly precise number for medical or research purposes, a DEXA scan remains the gold standard; this calculator is meant for practical, ongoing self-tracking with equipment you can own and use yourself.
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