Body Fat Calculator

The Navy formula

The method treats the waist (and hips, for women) as a proxy for fat and the neck as a proxy for lean mass, then combines them with height in a logarithmic formula. For men: body fat % = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450, with all measurements in the same units. A man 180 cm tall with a 40 cm neck and a 90 cm waist comes out at about 18.4%. The women's formula adds the hip measurement and uses different constants, reflecting different fat distribution.

The formula was validated against underwater weighing in military populations, and later studies put its typical error around three to four percentage points — good enough to track a trend, not precise enough to argue over a single point. Because it uses circumferences rather than weight, it separates a muscular build from a high-fat one in a way BMI cannot: two men of the same height and weight can have very different Navy results if one carries the weight at the waist and the other in the shoulders.

Measuring well matters more than the formula

The result is only as good as the tape. Measure the neck just below the Adam's apple, relaxed, with the tape level; the waist at the navel, relaxed at the end of a normal breath out — not sucked in, not over a thick jumper; and the hips at their widest point. Take each measurement twice and average them, because a single centimetre of error on the waist shifts the result by roughly a percentage point.

Measure under consistent conditions if you are tracking change: same time of day, same stomach contents, same posture. Waist circumference drifts through the day with food, water and bloating, and a morning measurement after the bathroom is the most repeatable baseline most people can manage.

Reading the result — and its limits

Commonly cited ranges put essential fat at roughly 2–5% for men and 10–13% for women, athletes below about 14% and 21% respectively, and average adults in the region of 18–24% for men and 25–31% for women — but these bands come from fitness references, not medical cut-offs, and healthy percentages vary with age, sport and genetics. A figure slightly above a band is not a diagnosis, and chasing a very low percentage carries its own health risks, including hormonal disruption.

Where the Navy method struggles is at the extremes: very lean people, very muscular people and people with unusual fat distribution all get less reliable estimates, and the formula was developed on young military adults, so older adults should treat it with extra caution. For a precise figure, DEXA scanning is the current reference standard, with trained skinfold calipers a cheaper middle ground; for tracking whether a training or diet plan is working, this calculator's trend over months is genuinely useful.