An estimate from a formula, not a body composition measurement. Not medical advice.
Two ways to estimate it
The US Navy method uses circumferences at the neck and waist (and hips for women) together with height. All it needs is a tape measure, and because it reads shape it lands closer to reality than BMI does. The Deurenberg formula works from BMI, age and sex alone — quicker, but with a wider error. A large gap between the two usually means your build is some distance from average.
The scales differ by sex
Women carry more essential fat as a matter of physiology, so the same percentage falls in a different band. The grading here follows the ACSM scale with separate thresholds. Applying the male scale to a woman marks healthy people as over-fat, which is why comparing the raw numbers across sexes tells you nothing.
These are estimates, not measurements
DEXA, hydrostatic weighing and skinfold callipers all disagree with each other, and formulas disagree with all of them. Use one method consistently and watch the direction of travel over months; the trend is meaningful even when the absolute number is not.
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Frequently asked questions
Q. Why does the tape method need the neck?
Because it stands in for frame size. Subtracting neck from waist separates the part of the circumference that reflects fat from the part that reflects build.
Q. Where exactly do I measure the waist?
At the navel for men, and at the narrowest point for women — that is what the Navy formula was fitted to. Measuring somewhere else changes the answer by several points.
Q. Which should I trust when they disagree?
The tape method, generally. The BMI-based estimate cannot tell muscle from fat, so it reads high on muscular people and low on those who are lightly built but under-muscled.