Short answer
Add three skinfolds (mm) to get S, compute body density, then body fat = 495 ÷ density − 450 (Siri). A man aged 30 with S = 60 mm has density 1.0578 and 17.9% body fat; a woman aged 30 with S = 60 mm has 1.0440 and 24.1%.
1. Sites
- Men: chest, abdomen, thigh.
- Women: triceps, suprailiac, thigh.
2. Equations
S is the sum of the three skinfolds in millimetres.
- Men: density = 1.10938 − 0.0008267 S + 0.0000016 S² − 0.0002574 × age
- Women: density = 1.0994921 − 0.0009929 S + 0.0000023 S² − 0.0001392 × age
- Siri: body fat % = 495 ÷ density − 450
3. Worked examples
| Person | S | Density | Body fat |
|---|---|---|---|
| Man, age 30 | 60 mm | 1.0578 | 17.9% |
| Man, age 40 | 90 mm | 1.0376 | 27.0% |
| Woman, age 30 | 60 mm | 1.0440 | 24.1% |
| Woman, age 40 | 75 mm | 1.0324 | 29.5% |
4. Technique matters
Skinfold results depend heavily on technique and operator skill. Use the same sites, the same side of the body and the same person taking the measurement each time, and take the reading more than once.
Enter your three readings in the Body Fat Calculator (skinfold calipers mode). If you only have a tape, see the Navy method.
5. Frequently asked questions
Which sites does the 3-site method use?expand_more
Chest, abdomen and thigh for men; triceps, suprailiac and thigh for women.
What is the Siri equation?expand_more
Body fat % = 495 ÷ body density − 450.
Why do results vary between people measuring?expand_more
Skinfold readings depend on technique and operator skill.
What do I enter in the calculator?expand_more
Your age, sex, weight, height and the three skinfold readings in mm.