The Navy Body Fat Formula: How It Works and How Accurate It Is

The short answer: the Navy formula estimates body fat percentage from your height and a few tape measurements (neck and waist, plus hips for women). It’s fast and free, but not precise. In a study of 1,904 soldiers it read lower than DEXA by an average of 5.88 points in men and 4.39 in women, and in recruits it missed much of the fat lost over 8 weeks.

What the Navy formula is

The Navy formula is a regression equation developed at the Naval Health Research Center in San Diego. Researchers measured body fat in service members with a lab method, then found the combination of circumferences that best predicted it. In studies it’s usually called the Hodgdon equation.

The inputs are simple:

• Men: height, neck and waist.
• Women: height, neck, waist and hips.

The equation runs those measurements through logarithms, so nobody does it by hand. Our body fat calculator applies it in metric or imperial units. Weight isn’t part of the percentage at all; it only comes in afterwards, to turn the percentage into kilograms (or pounds) of fat and lean mass. That’s why a Navy-method calculator gives the same percentage whatever weight you type.

How to measure for the Navy formula

Most of the error you can control comes from the tape, not the math.

• Neck: just below the larynx (Adam’s apple).
• Waist, men: at the navel, relaxed.
• Waist, women: at the narrowest point of the torso.
• Hips, women: at the widest point of the glutes.
• Tape tension: snug against the skin, not compressing it, and never sucking in.

Measure in the same spots, at the same time of day. Writing down where the tape sat (for example, “navel” rather than “roughly the waist”) makes the next check comparable.

How accurate is it? What the studies found

Several validation studies have compared the Navy formula with lab methods. They’re all military or college samples, so they’re not a perfect match for a recreational lifter, but they’re the best data available.

Soldiers vs. DEXA

A 2025 study in the International Journal of Obesity measured 1,904 active-duty soldiers (643 of them women) with DEXA and with the 3-site Hodgdon equation, the Navy formula. The tape equation underestimated body fat. The mean difference from DEXA was 5.88 percentage points in men and 4.39 in women. A newer 1-site equation (Taylor-McClung) did slightly better in men (4.38) and about the same in women (4.59).

College men vs. a three-compartment model

A 2008 study in Dynamic Medicine compared 31 college-age men against a three-compartment model, a lab reference that combines more than one measurement. The Navy and Air Force equation had a total error of 5.2% fat, which the authors judged unacceptable. Underwater weighing and the BOD POD came in at 2.5% and 2.7%.

Middle-aged women vs. underwater weighing

In a 1995 Medicine and Science in Sports and Exercise study of 62 women aged 40–60, the Navy equation was close on average: 30.3% body fat, against 29.5% by underwater weighing. But averages hide individuals. Bland-Altman plots showed a lack of agreement between the equation and the reference for individual women.

Where it errs most

Circumference equations share a pattern. In a 2022 Frontiers in Physiology study of 430 men and 179 women in the US Marines, a tape equation overestimated lean people and underestimated people with higher body fat. The authors called it a field method for screening that does not provide research-grade body composition data. Errors also change who gets flagged: a 1993 Military Medicine study of 49 men and 50 women who’d been flagged as over the limit found a 6.8 to 18% false-positive rate.

Can the Navy formula track fat loss?

This is the question most people actually have, and the best study on it is not encouraging over short periods.

A 2023 study in Frontiers in Physiology measured 481 women and 926 men at the start and end of 8 weeks of Army basic training, with both DEXA and the circumference equations. At the start, the tape read lower than DEXA by an average of 6.0 percentage points in both sexes. Then came the change:

The authors concluded that the tape underestimated the change in men and didn’t detect it in women, and that circumference-based body fat may not be an appropriate tool for tracking change over short training periods.

If you use the formula anyway, read your raw tape numbers too. A waist that’s shrinking while your weight holds steady is a real signal on its own, as covered in losing inches but not weight.

How the Navy formula compares with other methods

The Navy formula sits with skinfold calipers and smart scales as a field method: cheap, quick, and several points off for one person. For the side-by-side numbers on DEXA, calipers, bioimpedance and photos, see how accurate each body fat method is. If you’re weighing a lab scan against something you can repeat at home, DEXA scan alternatives covers the trade-offs.

In practice:

• For one accurate number, a lab method beats any tape equation.
• For a cheap baseline, the Navy formula is fine if you accept a few points of error.
• For weekly change, combine signals: your weight trend, raw waist measurements and consistent progress photos, checked about as often as change can show up.

Corpus is built for that last case: a weekly photo check-in with matched framing, pose, distance and light, read as a trend next to your weight. It doesn’t need a tape, and it doesn’t claim lab accuracy either.