Body Fat Calculator

The US Navy circumference method estimates body fat from a tape measure and a height. It is the cheapest useful method there is, and its accuracy is dominated by one thing you control completely: where you put the tape.

The metric formulas, worked through

The equations were developed by Hodgdon and Beckett at the Naval Health Research Center in 1984 as a field alternative to hydrostatic weighing. This tool uses the metric form, with every measurement in centimetres:

Men:   495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450
Women: 495 / (1.29579 - 0.35004 x log10(waist + hip - neck) + 0.22100 x log10(height)) - 450

Man, 180 cm, neck 38 cm, waist 90 cm. The girth term is 90 - 38 = 52, and log10(52) = 1.71600, so 0.19077 x 1.71600 = 0.32736. The height term is log10(180) = 2.25527, so 0.15456 x 2.25527 = 0.34857. The bracket is 1.0324 - 0.32736 + 0.34857 = 1.05361. Then 495 / 1.05361 = 469.81, and 469.81 - 450 = 19.8%, which the American Council on Exercise scale places in the "Average" band for men.

Woman, 165 cm, neck 33 cm, waist 75 cm, hip 95 cm. The girth term is 75 + 95 - 33 = 137, and log10(137) = 2.13672, so 0.35004 x 2.13672 = 0.74794. The height term is log10(165) = 2.21748, so 0.22100 x 2.21748 = 0.49006. The bracket is 1.29579 - 0.74794 + 0.49006 = 1.03792. Then 495 / 1.03792 = 476.92, and 476.92 - 450 = 26.9%, again "Average" on the ACE scale for women.

Both examples are recomputed from the formulas above. Earlier versions of this page printed the imperial-constant version of the equations while the tool accepted centimetres, and the two disagreed with each other and with what the code returned. Run through those old constants, the woman in the second example came out at 53.7% — a figure that would have been alarming and was simply wrong.

Tape placement is the whole ballgame

Against a DEXA scan the method carries an error of roughly three to four percentage points. Sloppy measurement adds to that, and it adds fast. Move the man's waist reading from 90 cm to 92 cm and the estimate goes from 19.8% to 21.2% — one and a half points from two centimetres.

So measure the same way every time:

• Neck just below the larynx, tape sloping very slightly downward at the front, snug but not compressing.
• Waist for men at the navel; for women at the natural narrowest point, which is usually a few centimetres higher.
• Hips at the widest part of the buttocks, feet together.
• Stand relaxed, arms down, and read the tape at the end of a normal breath out. Do not suck in, and do not pull the tape tight enough to dent the skin.
• Take each measurement three times and use the middle value.

Measure at the same time of day, before eating and before training. Abdominal girth changes across a day with food and fluid, and post-exercise swelling shifts it further.

What the number can and cannot support

A single reading is a rough estimate with a wide band around it. A series of readings taken identically over eight to twelve weeks is genuinely useful, because the measurement error largely cancels and the trend is real even when the absolute value is off. Track the direction, not the decimal.

The method was validated on adults of ordinary build and degrades at the extremes — very lean, very heavy, or unusually proportioned people fall outside the sample it was fitted to. It is meaningless in pregnancy, when abdominal girth reflects the pregnancy rather than fat distribution. And it is not a clinical assessment: DEXA remains the reference standard, with air displacement plethysmography and hydrostatic weighing close behind.

If your BMI and your body fat estimate tell different stories, that discrepancy is informative in itself — the BMI calculator explains why weight and shape can diverge. To turn a composition goal into a daily protein figure, the protein intake calculator works from body weight and training goal.

Nothing is uploaded; the arithmetic happens in your browser, and your measurements may be kept by your own browser so you do not retype them. No one with medical training has reviewed this page.