Estimate body fat percentage using Navy method from waist, neck, hip, and height measurements.
This calculator estimates body fat percentage from tape measurements and height, using the U.S. Navy circumference method, the same equations the armed services use for their tape test. Men enter height, neck, and waist measured at the navel; women enter height, neck, waist at its narrowest point, and hips at their widest. The result is a prediction, not a measurement: circumference equations estimate body density and carry an error band of several percentage points against DEXA or hydrostatic weighing. What surprises most people is how much tape placement matters - an inch of error moves the answer one to two points.
A concrete input and expected output from the current implementation.
Input
Female, 65 in tall, neck 12 in, waist 28 in, hip 38 in
Expected output
Estimated body fat: 27.3%
The women's Navy equation is 163.205 * log10(waist + hip - neck) - 97.684 * log10(height) - 78.387, all in inches. Here waist + hip - neck = 54, log10(54) = 1.7324, and log10(65) = 1.8129, so the result is 163.205 * 1.7324 - 97.684 * 1.8129 - 78.387 = 282.735 - 177.093 - 78.387 = 27.26, which rounds to 27.3%.
The tool applies the published Navy equations, which take base-10 logarithms of circumference combinations and height. For men: 86.010 x log10(waist - neck) - 70.041 x log10(height) + 36.76. For women: 163.205 x log10(waist + hip - neck) - 97.684 x log10(height) - 78.387. The constants assume inches throughout, so any centimeter input must be converted to inches before the logarithms are taken.
The equations only behave if the tape lands on the official sites. Men measure the abdomen horizontally at the navel, not the trouser-line waist. Women measure the waist at its narrowest point and the hips at their widest. The neck is measured just below the larynx. Because the formulas take logarithms of small differences, an inch of misplaced tape changes the result by roughly one to two percentage points.
The two methods estimate different things. This tool predicts body density from circumference ratios, while impedance scales infer fat from electrical resistance, which shifts with hydration. Both carry an error band of several percentage points, so a gap of three or four points is normal. Watch the direction of change over weeks, not the single number.
The Army, Navy, and Marine Corps use the same equations for their official tape tests, so the math matches. But official screens follow strict measurement procedures, and each service sets its own age- and sex-based maximum percentages, which are revised over time. For a job, waiver, or recruitment decision, check the current regulation for your service rather than relying on the number alone.
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