US Navy body fat calculator
An estimate of your body fat percentage from a tape measure: your height, neck and waist, plus your hips if you are a woman. James A. Hodgdon and Marcie B. Beckett of the Naval Health Research Center in San Diego developed it for the US Navy, publishing separate equations for men and for women in 1984, fitted on Navy personnel whose body fat had been measured by underwater weighing.
Where to hold the tape
Almost every complaint that a circumference calculator is wrong turns out to be a measuring problem rather than an arithmetic one. The three points below are the ones the equation expects, and getting them consistently is worth more than getting them perfect.
The equation itself
There is no reason to hide it. Both forms work in centimetres here, take a base ten logarithm of a circumference difference, and subtract a height term.
Men
495 / (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450Women
495 / (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450Two things follow from the shape of it. A waist very close to your neck measurement sends the logarithm toward zero and the whole thing has no sensible answer, which is why the calculator says so instead of printing a number. And because the only body inputs are circumferences, the equation cannot tell a muscular midsection from a soft one.
What it is good at, and what it is not
It is good at being repeatable. A tape measure costs almost nothing, the measurement takes a minute, and if you take it the same way each month the direction it moves is real information about you.
It is not good at being an absolute truth. It was fitted on US Navy personnel, so it assumes a build and a fat distribution close to that group, and it has no way of knowing whether your waist measurement is fat or muscle. Run the same numbers through Relative Fat Mass and you can see several percentage points of difference for the same person on the same day. That spread is not a fault, it is what estimating body composition from a tape measure actually looks like.
Common questions
How accurate is the Navy body fat calculator?
It lands within a few percentage points of a DEXA scan for most people, which is good enough to follow a direction and not good enough to argue with a scan. Its error is largest for the very lean and the very heavy, and for anyone whose fat sits differently from the military population the equation was fitted on. Measure yourself the same way each time and watch your own trend rather than the absolute number.
Why does the Navy method need my neck measurement?
Because it is trying to separate fat from everything else, and the neck is a part of you that carries relatively little fat. Subtracting the neck from the waist gives the equation a rough read on how much of your girth is fat rather than frame. It is the reason this method is more informative than one that uses the waist alone.
Why do women need a hip measurement and men do not?
There are two separate equations, fitted separately on men and on women. The women's equation uses waist plus hip minus neck, because fat distribution differs and the hip carries information for women that the waist alone does not. Without a hip measurement the female equation has no solution, so the calculator says so rather than substituting the male one.
Is this the same as the Army or Air Force body fat standard?
No, and it is worth not confusing them. This is the Navy circumference equation published by Hodgdon and Beckett in 1984. The other services have used their own procedures and their own coefficients, and each branch sets its own allowable percentages by age. Nothing on this page is a fitness standard for any service.
My result seems too low compared with other calculators
That is a known and expected difference between methods rather than a fault in any of them. Run the same measurements through Relative Fat Mass and the Deurenberg equation and you can easily see eight points of spread, because each was built from different measurements on different people. The comparison is on the body fat page.
Where exactly do I hold the tape?
Neck: just below the larynx, tape sloping very slightly down at the front. Waist: men at the navel, women at the narrowest point of the torso. Hip: around the widest part of the buttocks. Stand upright, breathe out, and keep the tape snug without compressing the skin. Consistency between measurements matters more than finding the theoretically perfect spot.
Sources
Every figure above comes from one of these. Check them.
- Hodgdon JA, Beckett MB (1984). Prediction of percent body fat for U.S. Navy men from body circumferences and height. Naval Health Research Center, San Diego, Report No. 84-11
- Hodgdon JA, Beckett MB (1984). Prediction of percent body fat for U.S. Navy women from body circumferences and height. Naval Health Research Center, San Diego, Report No. 84-29
- Gallagher D et al. (2000). Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. American Journal of Clinical Nutrition 72(3), 694-701
- Woolcott OO, Bergman RN (2018). Relative fat mass (RFM) as a new estimator of whole-body fat percentage. Scientific Reports 8, 10980
Take the same three measurements next month
The app keeps them, so the only number that matters, the direction yours is moving, is there without you working it out.
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This is not medical advice. Body Numbers is a calculator. It does not diagnose, treat or monitor any condition, and no number it produces should replace a conversation with a doctor or a dietitian. If a result worries you, or if you are pregnant, unwell, or under eighteen, speak to a health professional.
