The formula
US Navy: 495 ÷ (1.0324 − 0.19077 × log(waist − neck) + 0.15456 × log(height)) − 450
Where it comes from
The circumference method works because fat distributes predictably enough that the difference between waist and neck measurements tracks total body fat. It was developed by the US Navy for fitness screening, which tells you both its strength — cheap and repeatable — and its limitation — built for a young, fit population.
How to work it out by hand
- Measure the neck below the larynx, the waist at the navel and, for women, the hip at its widest
- Take the difference between waist and neck (women add the hip)
- Apply the logarithmic formula with your height
- Multiply by your weight for fat mass, and subtract for lean mass
What is worth knowing
The measurement is where the error lives, not the formula. A centimetre out on the waist moves the result by around a point, so measure the same way every time: same moment of the breath, tape snug but not compressing, same time of day. The absolute number is worth little on its own; the trend across weeks is worth a lot. For context, essential fat is about 3 to 5 % for men and 10 to 13 % for women, athletes typically sit at 6 to 17 % and 14 to 24 %, and the health risk climbs above roughly 25 % and 32 %.