Body Fat Calculator
Estimate body fat percent from tape measurements using the US Navy circumference method, developed by Hodgdon and Beckett at the Naval Health Research Center in 1984. All you need is a flexible tape.
Members: the Phorme app recalculates this automatically as you log.
A tape measure and two log curves.
The US Navy needed a body fat method that thousands of sailors could run with nothing but a tape measure. Hodgdon and Beckett built it at the Naval Health Research Center in 1984 by regressing circumference measurements against underwater weighing, the reference method of the era.
The result is a pair of logarithmic equations. Men: body fat is estimated from height and the waist minus neck difference. Women: from height and waist plus hip minus neck, because female fat distribution concentrates at the hips and a two-site version underestimates it.
Against lab methods the standard error runs 3 to 4 percentage points, so treat any single reading as a range. Its real strength is repeatability: measured the same way at the same time of day, it tracks your own trend more consistently than consumer bioimpedance scales.
- 1. Measure your neck just below the larynx with a flexible tape, level with the floor.
- 2. Measure your waist: at the navel for men, at the narrowest point for women.
- 3. Women: measure hips at the widest point. The female equation requires it.
- 4. Enter height and the measurements, then press Calculate for your body fat percent.
- 5. Re-measure every 2 to 4 weeks, same time of day, same tape tension.
Men: 495 / (1.0324 minus 0.19077 x log10(waist minus neck) + 0.15456 x log10(height)) minus 450, all in cm (Hodgdon and Beckett, 1984).
Women: 495 / (1.29579 minus 0.35004 x log10(waist + hip minus neck) + 0.221 x log10(height)) minus 450, all in cm (Hodgdon and Beckett, 1984).
Questions we hear a lot.
How accurate is the Navy circumference method?
Against underwater weighing, the original Hodgdon and Beckett work reported standard errors around 3 to 4 percent body fat. That is too coarse to compare two different people, but because the error stays consistent for one person measured the same way, it tracks your own change well.
How do I measure correctly?
Use a flexible tape held level with the floor and snug without compressing the skin. Neck: just below the larynx. Waist: at the navel for men, at the narrowest point for women. Hips: at the widest point. Stand relaxed, breathe out normally, and take each site twice.
Why does the female equation need a hip measurement?
Women carry proportionally more fat mass at the hips and thighs, so neck and waist alone miss the pattern. The three-site female equation was validated with hips included and is not defined without them.
Why is my result different from my smart scale?
Bioimpedance scales estimate from an electrical signal that shifts with hydration, food timing, and skin temperature, so readings can swing 2 to 4 points within a day. Tape measurements drift far less. Whichever tool you use, keep it constant and read the trend, not the single number.
What is a normal body fat percentage?
Broad reference ranges put most healthy trained adults around 10 to 20 percent for men and 18 to 28 percent for women. Athletes often sit below those ranges in season, and there is no health or performance prize for holding single digits year round.
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