Body Fat Calculator: US Navy Tape Measure Method

The US Navy formula

The circumference method was developed by Hodgdon and Beckett at the Naval Health Research Center in 1984 and is still used for military body composition screening. It predicts body fat from how much of your girth is at the waist relative to your neck and height. In inches: 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 logic is sensible. Fat collects around the waist (and hips for women), while the neck mostly reflects frame and muscle. A bigger waist-to-neck gap at the same height means more fat.

Worked examples

Man, 70 inches tall, neck 15.5 inches, waist 35 inches, 180 lb. Waist minus neck is 19.5. 86.010 × log10(19.5) = 110.9; 70.041 × log10(70) = 129.2. 110.9 − 129.2 + 36.76 = 18.5 percent body fat. At 180 lb that is about 33 lb of fat and 147 lb of lean mass, which ACE classifies as average for men.

Woman, 64 inches tall, neck 13, waist 30, hips 38. Waist plus hips minus neck is 55. 163.205 × log10(55) = 284.0; 97.684 × log10(64) = 176.4. 284.0 − 176.4 − 78.4 = 29.2 percent, in the average band for women (25 to 31 percent).

Measuring correctly matters more than the formula

A half-inch error at the waist moves the result by one to two percentage points, so technique beats precision equipment. Use a flexible, non-stretch tape. Measure the neck just below the larynx, sloping slightly down toward the front. Men measure the waist horizontally at the navel; women at the narrowest point of the torso. Women measure hips at the widest point of the buttocks.

Stand relaxed, breathe out normally, and don't suck in. Keep the tape snug against the skin without compressing it. Take each measurement two or three times and use the average. For tracking, measure at the same time of day, ideally in the morning before eating.

What the categories mean

The American Council on Exercise groups results like this. Men: essential fat 2 to 5 percent, athletes 6 to 13, fitness 14 to 17, average 18 to 24, obese 25 and above. Women: essential fat 10 to 13 percent, athletes 14 to 20, fitness 21 to 24, average 25 to 31, obese 32 and above.

Women carry more essential fat for hormonal and reproductive functions, which is why every band sits higher. Going below essential fat levels is dangerous for anyone, and very low numbers in a result usually point to a measurement error rather than an extraordinary physique.

How it compares with other methods

DEXA scans, hydrostatic weighing, and air displacement (Bod Pod) are the lab standards, and each has its own error of a couple of points. The Navy method generally lands within a few points of them for typical adults but can misjudge people with unusual fat distribution or very thick necks from training.

Smart scales use bioelectrical impedance, which swings with hydration: the same person can read three or four points apart morning and evening. Skinfold calipers are accurate in trained hands and inconsistent otherwise. The BMI-based estimate shown under the result uses the Deurenberg (1991) formula and needs no tape, but it can't see muscle, so a muscular person will read high.

Using the number well

Track the trend, not a single reading. Measure every two to four weeks under the same conditions. If your weight is flat while your waist shrinks, you are likely losing fat and gaining muscle, a good result a scale alone would miss. Fat mass and lean mass in pounds make this concrete.

Waist size matters independently. A waist above 40 inches for men or 35 inches for women is linked to higher cardiometabolic risk regardless of body fat percentage. This tool is an educational estimate, not a medical assessment, and your measurements never leave your browser.