Body composition calculator
Jackson-Pollock 4-site calculator
Body fat from four skinfolds — triceps, suprailiac, abdomen and thigh — using the Jackson, Pollock & Ward equation for women. Free, instant, no sign-up.
This equation was fitted on women. If you are male, use the 3-site or 7-site calculator, or Durnin & Womersley — see below.
Skinfolds track change well. The coach turns that trend into a plan.
What is the Jackson-Pollock 4-site method?
It is a skinfold equation published by Jackson, Pollock and Ward in 1980 that predicts body density from four caliper measurements — triceps, suprailiac, abdomen and thigh — plus age. The density is then converted to a body-fat percentage. Four sites is the middle of the range: fewer folds than the seven-site method, so it is quicker and there are fewer places to go wrong, but more than three, so a single badly taken fold matters less.
Why this equation is for women
The 1980 paper fitted this equation on a sample of 249 women, and the four sites were chosen because they predicted density best in that sample. Running a man's folds through it would not be an approximation — it would be a different regression applied to the wrong population. Jackson and Pollock's 1978 men's paper published three-site and seven-site equations, not a four-site one. The four-fold method people usually mean for men is Durnin and Womersley's, which uses biceps, triceps, subscapular and suprailiac, and has its own calculator here. Rather than invent a male row for a table that does not have one, this page sends men to the equation that was actually fitted on them.
How this calculator works
The four folds are summed and run through the quadratic that Jackson, Pollock and Ward fitted: body density equals 1.096095 minus 0.0006952 times the sum, plus 0.0000011 times the sum squared, minus 0.0000714 times age. The age term is not decoration — subcutaneous fat makes up a smaller share of total fat as people get older, so identical folds twenty years apart genuinely correspond to different body compositions. The resulting density is converted to a percentage with Siri's equation, and Brožek's is shown alongside it, because that conversion is a separate modelling choice and the two disagree by about a percentage point at the extremes.
Taking the folds
All four are taken on the right side of the body. Triceps: a vertical fold on the back of the upper arm, midway between the shoulder and the elbow. Suprailiac: a diagonal fold just above the hip bone, following the natural line of the skin. Abdomen: a vertical fold about 2 cm to the side of the navel. Thigh: a vertical fold on the front of the thigh, midway between the hip and the knee. Pinch the skin and fat, pull it away from the muscle underneath, apply the calipers a centimetre below your fingers, and read after two seconds. Take each site three times and use the median, not the average.
How accurate is it?
The standard error of a well-taken skinfold estimate is around 3.5 percentage points against a laboratory criterion, which is good for the equipment involved but wide enough that a single reading should not be treated as a precise number. Most of the error in practice is not the equation — it is the person holding the calipers. The same tester measuring the same person twice gets closer results than two testers measuring once each, which is why skinfolds are excellent for tracking change over time and mediocre for a one-off verdict. Measure the same way, at the same time of day, ideally with the same hands.