How it works
What skinfold thickness actually represents
About half of your body fat sits just under the skin (subcutaneous) and half wraps around your organs (visceral). Skinfold calipers measure the subcutaneous half by pinching a fold of skin and fat away from the underlying muscle, then clamping it with a calibrated spring. The thickness at that site correlates very tightly with total body fat across large populations — which is why Jackson & Pollock built their 3-site equations around skinfold sums.
The three sites, step by step
Pick the right protocol for sex, because the equations are sex-specific. All measurements go on the right side of the body with the subject relaxed and standing upright.
Men: chest · abdomen · thigh
Chest — diagonal fold halfway between the nipple and the front of the armpit.
Abdomen — vertical fold 2 cm to the right of the navel, at the level of the belly button.
Thigh — vertical fold on the front of the thigh, midway between the hip crease and the top of the kneecap.
Women: triceps · suprailiac · thigh
Triceps — vertical fold on the back of the arm, halfway between the tip of the shoulder and the elbow.
Suprailiac — diagonal fold on top of the hip bone, following the natural angle of the iliac crest.
Thigh — same mid-thigh site as for men, vertical.
Why technique matters more than the caliper brand
A study in *Medicine & Science in Sports & Exercise* found that between-tester variability on the same person is typically 3–4 mm per site for untrained assessors but drops to under 1 mm for ISAK-certified technicians. That means training and consistency matter more than whether you use a Harpenden (£300) or a Slim Guide (£30). Three tips that collapse most of the error:
- Always measure on the right side, with the subject relaxed — clenched muscles artificially reduce the fold.
- Pinch firmly, lift the fold away from the muscle, and close the caliper 1 cm below your pinch.
- Wait 2 seconds before reading — the caliper jaws continue to compress for a moment.
- Take two readings per site and average; re-measure if they differ by more than 2 mm.
Turning the sum into body-fat percentage
The calculadora sums your three skinfolds, plugs the total and your age into the Jackson-Pollock density equation, and then feeds that density into Siri: %body fat = 495 / density − 450. You see all three outputs — sum, density, body-fat % — so you can track them independently across time.
How often to retest
Every 4 to 6 weeks is the sweet spot. Weekly testing picks up too much day-to-day noise from hydration and glycogen, and longer than two months misses the feedback window most people need. Test at the same time of day, in the same hydration state (mid-morning after normal breakfast works well), and have the same person take every reading.