Skinfold Body Fat (Durnin-Womersley) Calculator

How it is calculated

The four skinfolds (biceps, triceps, subscapular, suprailiac) are added in millimetres and entered as one sum. Body density is density = c − m × log₁₀(sum), where the constants c and m depend on sex and on the age band (under 20, 20–29, 30–39, 40–49, 50 and over). The Siri equation then converts it: body fat % = 495 ÷ density − 450. The category compares body fat with sex-specific limits: men under 14% Athletic, under 25% Average, otherwise Above average; women under 21%, under 32%, otherwise Above average. The tool shows no result when age is zero or above 130, when the sum is so small that the fat estimate would be zero or negative (a 5 mm sum for a 35-year-old man), or when the estimate would exceed 75%. Any sex value other than female is treated as male. It is an estimate; caliper technique and the person measuring matter.

Common mistakes

• Entering one skinfold instead of the sum of all four sites. A single 10 mm fold is read as a sum of 10 mm, which gives a body fat of 5.05% and an Athletic label for the same 35-year-old man.
• Entering centimetres or inches; the sum must be in millimetres. A sum of 4 (cm) is far too small and shows no result for a woman.
• Using a different protocol's sum (for example a 3-site or 7-site total), because the coefficients are built only for the biceps-triceps-subscapular-suprailiac sum.
• Choosing the wrong sex or ignoring the age band. The constants change at 20, 30, 40 and 50, so the same 40 mm sum gives 19.17% at age 39 but 21.49% at age 40 for a man.