Body composition calculator
Durnin & Womersley calculator
Body-fat percentage from four upper-body skinfolds — the Durnin & Womersley method, published in 1974 and still standard in clinical work. Free, instant, no sign-up.
All four sites are on the upper body, so this one can be taken without undressing.
One measurement is a baseline — the coach turns it into a plan and re-checks it as you go.
What is the Durnin & Womersley method?
John Durnin and Josephine Womersley measured four skinfolds and whole-body density by underwater weighing on 481 people aged 16 to 72, and published the resulting equations in the British Journal of Nutrition in 1974. It is the oldest skinfold method still in routine use, and the one you are most likely to meet in a clinical or research setting rather than a gym — partly because all four sites are on the upper body, so it can be taken through a rolled-up sleeve.
How this calculator works
Biceps, triceps, subscapular and suprailiac are summed in millimetres, and the density comes from D = c − m·log₁₀(Σ), where c and m are read from a table by sex and age band. For a woman aged 20 to 29 that is 1.1599 − 0.0717·log₁₀(Σ). Siri's equation, 495/D − 450, converts the density into a percentage. Note the logarithm: unlike Jackson & Pollock's quadratic, this one flattens out as the folds get thicker, which is why the two methods drift apart at higher body-fat levels.
Why the estimate jumps on a birthday
Durnin & Womersley fitted a separate regression for each age band — 17-19, 20-29, 30-39, 40-49, and 50 and over — rather than putting age into the equation as a continuous term the way Jackson & Pollock did. The consequence is real and worth knowing about: the same four folds can read a point or two differently on either side of a band boundary. It is an artefact of how the equations were published, not a change in your body, so if you cross a boundary mid-series, note it and keep going.
How to take the folds
Measure the right side with the arm hanging loose and the muscle relaxed. Biceps is a vertical fold on the front of the upper arm, midway between shoulder and elbow; triceps is the vertical fold at the same height on the back; subscapular is a diagonal fold running down and out from just below the bottom corner of the shoulder blade; suprailiac is a diagonal fold just above the crest of the hip bone, following the natural line of the skin. Pinch about a centimetre away from where the jaws will sit, release the caliper fully and read after two seconds, and take three readings per site in rotation — use the median, not the mean.
How to use the number
The 1974 sample was British and largely sedentary, so the equations tend to read high on very lean or heavily trained people. That matters less than it sounds: the value of any skinfold method is the trend it draws over months, and a consistent bias cancels out of a trend. Keep the same tester, the same sites and the same method, and treat the number as an estimate for training decisions rather than a medical assessment.