How it works
Why density, not weight, is the better fitness signal
Two people can weigh the same and look completely different on a scan — muscle tissue is about 1.10 g/cc and fat is about 0.90 g/cc, so a lean, muscular athlete packs more mass into less space than a sedentary adult of the same weight. Body density captures that difference in a single number, which is why sports scientists built the Jackson-Pollock equations around it.
You do not need a lab or a DEXA scan to estimate density. A £30 set of skinfold calipers, a tape measure and a calculadora like this one will land you within about 3.5% of hydrostatic weighing for most adults, as long as the technique is consistent.
The Jackson-Pollock 3-site equations
Published by Jackson & Pollock in 1978 (men) and Jackson, Pollock & Ward in 1980 (women), these equations are the most widely taught skinfold method in ACSM-certified training and university exercise-science courses.
| Population | Sum of sites | Density equation |
|---|---|---|
| Men | Chest + abdomen + thigh | D = 1.10938 − 0.0008267·ΣSF + 0.0000016·ΣSF² − 0.0002574·age |
| Women | Triceps + suprailiac + thigh | D = 1.0994921 − 0.0009929·ΣSF + 0.0000023·ΣSF² − 0.0001392·age |
From density to body fat: the Siri equation
Once you have density, plug it into William Siri's 1961 two-compartment model: %body fat = (495 / density) − 450. Example: a density of 1.0650 g/cc gives 495 / 1.0650 − 450 ≈ 14.8% body fat. For very lean athletes the Brozek variant is marginally more accurate, but for the general population Siri is the accepted default.
How to measure skinfolds consistently
Use a calibrated caliper (Harpenden, Lange or Slim Guide) and take every site on the right side of the body. Pinch the fold of skin and subcutaneous fat firmly, lift away from the muscle, and close the caliper jaws 1 cm below your pinch. Wait two seconds before reading — the jaws continue to compress for a moment.
- Chest (men): diagonal fold, halfway between the nipple and the anterior axillary line.
- Abdomen (men): vertical fold 2 cm to the right of the navel.
- Thigh (both): vertical fold on the front of the thigh, midway between the hip crease and top of the kneecap.
- Triceps (women): vertical fold on the back of the arm, halfway between the shoulder and elbow.
- Suprailiac (women): diagonal fold on the top of the hip bone, in line with the natural angle of the iliac crest.
What the result tells you
Density is a mid-step — the number you actually care about is body-fat percentage, and from there you can back-calculate fat mass and fat-free mass by multiplying by your scale weight. Retest every four to six weeks, always at the same time of day and hydration state. Hydration swings alone can nudge a single skinfold reading by 1–2 mm.