Body Fat Calculator (US Navy)

US Navy body-fat method

The US Navy Body Composition Program works out your body-fat percentage from a tape measure alone. You don't need calipers or a scale. Men: %BF = 86.010·log10(waist − neck) − 70.041·log10(height) + 36.76. Women: %BF = 163.205·log10(waist + hip − neck) − 97.684·log10(height) − 78.387. The original formulas were written for inches; this calculator takes centimeters and handles the conversion for you. Measure the neck just below the larynx, the waist at the navel for men and at its narrowest point for women, and the hip at its widest. As for accuracy, expect roughly 3-4 percentage points of error against DEXA, which is about what you'd get from skinfold calipers used by someone who knows what they're doing. The method tends to underestimate very lean bodybuilders, since their waist is small relative to neck and height, and it can overestimate heavier, abdominally built non-athletes. Example: a man of 175 cm with a 38 cm neck and 85 cm waist comes out at about 17% body fat.

Applications: military, gyms and body recomp

The US Navy uses it to check fitness compliance, and the Brazilian Armed Forces adapted it for the FCB (Forças Armadas) physical assessments. You'll also see it in gyms as a free stand-in for DEXA or bioimpedance, and people doing body recomposition use it to track themselves. The trend over a few weeks tells you more than any single reading. It won't diagnose obesity in a clinical sense, but as a screening and progress tool it does the job.

FAQ

How accurate is it vs DEXA? The typical error sits around 3-4 percentage points. It's good for watching change over time but shouldn't be read as a precise clinical number.

Why does it underestimate lean bodybuilders? The formula bakes in a normal waist-to-neck ratio. A thick, muscular neck paired with a tiny waist drags the result below the real %BF.

Do I need a flexible tape? Yes. A non-stretch fabric or fiberglass tape works best. Keep it parallel to the floor and snug, without digging into the skin.

How often should I measure? Once every 2-4 weeks at the same time of day (morning and fasted is best) gives you a clean trend without the noise of daily fluctuation.