Two people of identical height and weight can have completely different body compositions, and completely different health risks. Body fat percentage separates them.
This calculator uses the US Navy method, which estimates body fat from tape measurements: neck and waist for men, plus hips for women. It was developed for military assessment and remains one of the more practical approaches without specialist equipment.
Rough bands for men: 2 to 5 per cent is essential fat, 6 to 13 athletic, 14 to 17 fit, 18 to 24 average, and 25 or above obese.
For women the scale sits higher, because women carry more essential fat: 10 to 13 per cent essential, 14 to 20 athletic, 21 to 24 fit, 25 to 31 average, and 32 or above obese.
Those differences are physiological, not a matter of fitness, and comparing a woman's number to a man's is meaningless.
The Navy method is typically within 3 to 4 percentage points of a DEXA scan, which is the reference standard. That is adequate for tracking yourself and inadequate for precise claims.
Accuracy depends heavily on measuring the same way each time. Tape position, tape tension, whether you have eaten, and time of day all shift the result by more than the change you are trying to detect.
The method also assumes a typical fat distribution, so it is less accurate at the extremes: very lean, very heavy, or unusually muscular.
Treat it as a trend line rather than a fact. A change from 22 to 19 per cent measured the same way is meaningful. A single reading of 19 is an estimate.
Neck, just below the larynx, tape sloping slightly downward at the front.
Waist, at the navel for men, at the narrowest point for women.
Hips, for women, at the widest point.
Stand relaxed, breathe out normally, and keep the tape snug without compressing the skin. Morning, before eating, is the most consistent time.
Lower is not automatically better. Very low body fat impairs hormone production in both sexes: in men it suppresses testosterone, and in women it can stop menstruation entirely.
For most people, the useful goal is a healthy range alongside adequate muscle mass, rather than the lowest number achievable. FFMI is the companion measure for that, since it describes lean tissue rather than fat.
Related reading: FFMI calculator · Waist-to-height ratio · How much muscle can you build naturally · When training stops your periods
Educational only, not medical advice.