A tape measure and three numbers. This is a measurement, not an assessment — we report what the method returns and how far it can be out.
Estimated body fat
US Navy circumference method — the method's own standard error is about ±3 percentage points
Fat mass30.1 lb
Lean body mass145.9 lb
Likely range14.1% to 20.1%
Waist minus neck
—
Standard error
±3 percentage points vs hydrostatic weighing
Measured again in four weeks
A percentage point takes two to four weeks to move. Compare monthly, not weekly.
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Last checked
What you need to measure
Three measurements for men, four for women, all with a soft tape held snug but not tight, on bare skin, at the end of a normal breath out.
Height — without shoes.
Neck — just below the larynx, tape sloping slightly down at the front.
Waist — men measure at the navel; women measure at the narrowest point.
Hips — women only, at the widest point.
Measurement technique is the largest source of error in this method by a wide margin. Take each measurement three times and use the middle value; a half-inch difference at the waist moves the result by roughly a percentage point.
Worked example: man, 5 ft 11 in, neck 15 in, waist 34 in, 176 lb
At 176 lb that is 30.1 lb of fat mass and 145.9 lb of lean body mass. Given the method's error band, the honest reading is “somewhere between about 14% and 20%”.
Why this method and not a scale
The Navy circumference method has one significant advantage: it is repeatable. A tape measure gives you the same reading tomorrow morning that it gave you this morning, whereas a bio-impedance scale can move three percentage points depending on how hydrated you are and when you last ate. For tracking a change over eight weeks — which is what almost everyone actually wants — a repeatable method with a known bias beats a variable method with a smaller theoretical error.
It is also free, which DEXA and hydrostatic weighing are not.
Methodology
The US Navy circumference method, using the Hodgdon-Beckett equations developed for the US Department of Defense in 1984. Men: %BF = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76. Women: %BF = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387. All measurements in inches; metric inputs are converted before the equation is applied.
Limitations, stated plainly
The published standard error of this method is around ±3 percentage points against hydrostatic weighing, and individual results can be further out than that. It infers body composition purely from circumferences, so it systematically under-reads people who carry fat somewhere other than the waist, and over-reads people with thick waists made of muscle rather than fat. Very lean and very heavy people sit outside the population it was fitted on and get the worst accuracy.
What this number is not. It is a measurement of your body, in the same way a tape measure reading is. It is not an assessment of you, it does not carry a verdict, and we are not going to attach one. Competitive bodybuilders peak in the single digits; most people who train recreationally measure somewhere in the teens or twenties. Where you sit is information, not a grade.
This is a fitness estimate, not medical advice, and not a substitute for a professional who can actually see you.
Common questions
How accurate is the Navy body fat method?
Its published standard error is about ±3 percentage points compared with hydrostatic weighing, and individual readings can be further out. Where it earns its place is repeatability: measured the same way each time, it tracks change reliably, which is what most people actually want it for. Treat the absolute number as approximate and the trend as meaningful.
Why does my smart scale give a different number?
Because it is measuring something entirely different — the electrical resistance of your body, which depends heavily on hydration, recent food, recent exercise and skin temperature. Bio-impedance readings can swing three percentage points between morning and evening on the same person. Neither method is a gold standard; the tape at least gives you the same answer twice.
Where exactly do I measure my waist?
For men, at the navel, tape horizontal all the way round. For women, at the narrowest point of the torso, which is usually a little above the navel. Not where your trousers sit, and not pulled tight — snug enough that the tape stays put without compressing the skin. Measure at the end of a normal exhale, not holding your stomach in.
Why does the calculation use my neck?
The neck acts as a proxy for frame size and for the portion of your girth that is not fat. Subtracting it from the waist is what lets a single circumference say something about composition rather than just size. It is also the reason the method reads oddly for people with unusually thick or thin necks for their build.
How fast can body fat percentage change?
Slowly enough that weekly measurement is mostly noise. At a moderate rate of fat loss, a percentage point takes two to four weeks for most people. Measure every four weeks under identical conditions — same time of day, before eating, same tape, same technique — and compare those, not consecutive mornings.
Does this work for very lean or very muscular people?
Less well. Both groups sit outside the population the equations were fitted on. A very muscular person with a thick waist tends to read high, and a very lean person tends to read high too, because the equation has a floor it does not go far below. If you are at either extreme and the number matters, a DEXA scan is the answer — this method is not.