Skinfold Body Fat (3-Site) Calculator

How it is calculated

Enter the three skinfold thicknesses in millimetres and your age in years. The three boxes are labelled only 1st, 2nd and 3rd site, so use the order of the sites for your sex: men chest, abdomen, thigh; women triceps, suprailiac, thigh. With S = the sum of the three, body density is D = 1.10938 − 0.0008267·S + 0.0000016·S² − 0.0002574·age for men and D = 1.0994921 − 0.0009929·S + 0.0000023·S² − 0.0001392·age for women. Body fat is then given by the Siri equation %BF = 495 ÷ D − 450. Any sex other than female uses the male formula. Folds must not be negative and age must be above zero. The result is an estimate, not a medical measurement.

Common mistakes

• Measuring the wrong sites. The formula assumes chest, abdomen and thigh for men and triceps, suprailiac and thigh for women; entering, say, a biceps fold gives a number that looks valid but is wrong.
• Entering the fold in centimetres or inches. All three must be in millimetres; 1.5 cm is 15.
• Entering a single reading rather than the usual practice of repeated measurements. A fold taken once, or taken with a different pinch each time, makes the result jump by several percentage points; take each site again and average.
• Choosing the wrong sex. The two equations give clearly different answers for identical folds (13.61% against 19.06% here), so the setting matters.
• Reading the body fat as precise. It is a regression estimate from skin folds, so treat small week-to-week changes with caution and always measure at the same sites and time of day.