Body Fat % Calculator (US Navy Method)

The US Navy method estimates body fat from a tape measure and nothing else — no calipers, no scale that claims to read through your feet, no appointment. Enter height and neck and waist circumference, plus hips for women, in metric or imperial, and it returns body fat percentage along with the fat mass and lean mass those figures imply and a rough fitness category.

Underneath is a logarithmic regression fitted to circumference and height data from Navy personnel, not a measurement of anything. It works because the places fat is deposited correlate with girth at a few landmarks, and it fails wherever that correlation does — a heavily muscled neck pushes the estimate down, visceral fat at the waist pushes it up, and a very lean or very obese body sits at the edge of the sample the equation was fitted on. Against DEXA it lands within about three to four percentage points for most adults, which is fine for watching a trend and useless for a single headline number.

Measurement technique matters more than most people expect, and the arithmetic shows why. With a 178 cm man, one centimetre of difference at the waist moves the result by about 0.7 percentage points, and one centimetre at the neck moves it by roughly the same amount in the other direction. That is larger than a month of real change for most people, so a tape held at a different height, pulled tighter, or taken after a meal instead of before will produce a "result" that is entirely artefact. Measure at the same landmarks, at the same time of day, relaxed and not holding your breath, and only compare like with like.

Two things this number is not. It is not a health assessment: body fat percentage says nothing about where the fat sits, and visceral fat around the organs carries risk that subcutaneous fat does not, at the same total. And the Navy's own use of it is a compliance standard with published allowances, not a fitness target — the categories shown here are the common athletic and general-population bands, which are conventions rather than clinical thresholds.