Body composition calculator
Jackson-Pollock 3-site calculator
Body-fat percentage from three caliper readings and your age — the Jackson & Pollock three-site method. Free, instant, no sign-up.
Men measure chest, abdomen and thigh — the three sites the 1978 male equation was fitted on.
A caliper reading is a starting point — the coach turns it into a plan and re-checks it as you go.
What is the Jackson-Pollock 3-site method?
Andrew Jackson and Michael Pollock published their generalised skinfold equations for men in 1978, and Jackson, Pollock and Ann Ward published the female equations in 1980. The three-site version is the short form of their seven-site method: three folds instead of seven, picked because they carried most of the predictive weight in a sample of several hundred adults. Like every skinfold method it predicts body density rather than fat directly, and a second equation converts that density into a percentage.
How this calculator works
Men sum chest, abdomen and thigh; women sum triceps, suprailiac and thigh. That sum goes into an age-adjusted quadratic — for men, D = 1.10938 − 0.0008267·Σ + 0.0000016·Σ² − 0.0002574·age — and the density that comes out is converted with Siri's equation, 495/D − 450. Age is in there for a reason: subcutaneous fat makes up a smaller share of total fat as you get older, so identical folds read as more body fat at fifty than at twenty.
How to take the folds
Pinch skin and fat, not muscle. Roll the fold between thumb and forefinger about a centimetre away from where the caliper jaws will land, and check it does not tighten when the muscle underneath contracts. Chest is a diagonal fold halfway between armpit and nipple; abdomen is vertical, about 2 cm to the side of the navel; thigh is vertical, midway between hip crease and kneecap; suprailiac is diagonal, just above the hip bone; triceps is vertical, midway between shoulder and elbow with the arm hanging loose. Measure the right side, read the caliper two seconds after releasing it, take three readings per site in rotation rather than back to back, and use the median — the mean lets one bad pinch drag the whole number.
How to use the number
Skinfold equations carry a standard error of roughly 3.5 percentage points against hydrostatic weighing, and in practice most of the error is the person holding the calipers rather than the equation. The number is worth much more as a series than as a single reading: same tester, same sites, same time of day, every four to six weeks. If you switch to the seven-site version or to Durnin & Womersley, start a new series — the methods do not agree closely enough to compare across.