The Navy Body Fat Method: How to Measure It and How Wrong It Can Be
The US Navy needed a body composition screen that a petty officer could run on a few hundred sailors in an afternoon without equipment. Hodgdon and Beckett built it at the Naval Health Research Center in 1984 by regressing tape measurements against hydrostatic weighing, which at the time was the reference standard.
The result estimates body fat from circumferences and height. For men:
%fat = 495 / (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
For women the hip measurement joins in, because fat distribution differs enough that the male equation fails badly on female bodies:
%fat = 495 / (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
You do not need to run these by hand. The body fat calculator takes the measurements in centimetres or inches and handles the logarithms.
What the formula is really doing
Look at what it uses: the difference between your waist and neck, scaled against your height. It is measuring how much bigger your midsection is than your frame. That is a decent proxy for total fat in most people, and a poor one in anyone whose fat sits somewhere unusual.
It never asks your weight. Two people of the same height with identical tape numbers get the same result whether they weigh 70 kg or 95 kg. That independence from the scale is a genuine advantage over BMI, which cannot separate muscle from fat at all, but it comes with its own blind spot.
Measure it properly or do not bother
Tape placement is where most of the error comes from. A centimetre of drift on the waist moves the result by roughly half a percentage point.
- Neck. Just below the larynx, tape sloping slightly downward at the front. Do not flex or tuck your chin.
- Waist. Men measure at the navel. Women measure at the narrowest point, usually a little above. Take the reading at the end of a normal exhale, and do not pull your stomach in.
- Hip, women only. The widest point of the buttocks, tape level all the way round.
Keep the tape snug enough not to sag but not tight enough to dent the skin. Measure first thing in the morning before eating or drinking, take each measurement three times, and use the middle value. Measuring in the evening after a large meal can add 2 cm at the waist, which is worth a full percentage point of apparent body fat.
How accurate is it
Against hydrostatic weighing, the circumference method carries a standard error of roughly 3 to 4 percentage points. If it reads 18%, your true figure is probably between 14 and 22%. Compared with a DEXA scan it tends to read low on lean people and high on heavier ones.
Three groups get consistently poor results. Very lean athletes below 10% often read several points high. People carrying most of their fat on hips and thighs rather than the abdomen read low, since the waist measurement never sees it. And anyone with substantial abdominal muscle development reads high, because the tape cannot tell a thick oblique from a fat roll.
Use it for the trend, not the number
The absolute figure is soft. The direction is reliable, because the measurement errors mostly stay constant if you keep your technique identical. Someone tracking 22%, 21.4%, 20.7% over six weeks is losing fat, even if the true values are three points off across the board.
That makes it genuinely useful during a diet. Weight alone cannot tell you whether you are losing fat or muscle, and a stalled scale with a shrinking waist is a good outcome that looks like failure. Pair the tape with the weekly weight average described in how big a calorie deficit should be, and remember that a single morning’s reading on either measure is noise, for the reasons set out in why scale weight fluctuates.
Reasonable ranges
For men, roughly 6 to 13% is athletic, 14 to 17% fit, 18 to 24% average, and above 25% is where health risk climbs. For women the whole scale shifts up by about 10 points, because essential fat is higher: 14 to 20% athletic, 21 to 24% fit, 25 to 31% average, above 32% elevated risk. Women should treat anything below 14% as a warning sign rather than a goal.