Three or four caliper measurements convert to body density via the Jackson-Pollock or Durnin-Womersley equation, then to body-fat percentage via Siri. Track lean mass over a season, that’s the number that matters for endurance athletes, not weight on a scale.
Pick a method, enter your measurements in millimetres. Take each fold 3 times and use the average. Always measure on the right side of the body, on dry skin, before training (not after).
Skinfolds are technique-sensitive. Same caliper, same site, same person measuring, or the trend is noise.
Always measure on the right side of the body. Skin must be dry. Take the measurement before training, sweat and post-exercise hydration shifts can change the read by 1–2%.
Grip the skin between thumb and forefinger about 1 cm from the measurement site, lift away from the muscle, then place the caliper jaws on the fold. Read at 2–3 seconds, longer compresses the fold and gives a low read.
Two reads if they’re within 1 mm of each other. If the third differs by > 2 mm, take a fourth and median it. Trend matters more than absolute number, same caliper + same person measuring repeats well.
Caliper choice: Harpenden, Lange, Slim Guide are the three standards. Cheap plastic calipers can be off by ±2 mm. For tracking changes over a season, a single cheap caliper used by the same person each time gives you better data than rotating between expensive calipers.
Anatomical landmarks for each fold.
Diagonal fold, halfway between the anterior axillary line (front of armpit crease) and the nipple. Lift fold along the diagonal axis.
Abdomen (JP3 men): vertical fold ~2 cm right of the navel. Suprailiac (DW): diagonal fold just above the iliac crest along the anterior axillary line.
Vertical fold on the front of the thigh, midway between the inguinal crease (hip flexor crease) and the top of the kneecap. Standing weight on the other leg.
Vertical fold on the back of the upper arm, halfway between the acromion (shoulder bone) and the olecranon (elbow). Arm relaxed at the side.
Vertical fold at the front of the upper arm, halfway between the shoulder and the elbow, over the belly of the biceps with the arm relaxed.
Diagonal fold 1–2 cm below the inferior angle (bottom corner) of the scapula, running 45° down and laterally toward the elbow.
Two athletes lose 3 kg over a training block. Athlete A loses 1 kg fat + 2 kg muscle. Athlete B loses 3 kg fat + 0 kg muscle. The scale tells the same story; your performance won’t.
Lean mass is the proxy for the engine. If lean mass is dropping, you’re under-fuelling, under-recovering, or losing the wrong tissue. If lean mass holds or climbs while body fat drops, the work is doing what you want it to do.
For absolute precision, tracking 0.5% changes, pair calipers with DEXA every 6 months. Calipers track your trend; DEXA gives you the calibration.
Two equations, both validated against hydrostatic weighing on adult populations.
Three sites, sex-specific equations. Best for athletes, populations were lean and fit. Slight under-prediction for very lean athletes (< 10% men / < 16% women). Standard error ~3.5% body fat.
Four sites with 5 age brackets × 2 sexes coefficients. More robust across the general population, including older athletes. Slight over-prediction for very lean. Use it for non-elite or master’s populations. Standard error ~3.5%.
Both methods compute body density first; BF% = (495 / BD) − 450. Assumes fat density 0.9 g/cm³ and lean density 1.1 g/cm³, valid for most adult populations. Brozek alternative differs by < 1%.
DEXA at the lab gives you bone, lean, and fat by region with sub-1% precision. Track over a season with calipers; calibrate to truth twice a year with DEXA. The combination beats either alone.
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