MyGymCalcWhat is body fat percentage?
Body fat percentage is the share of your total weight made of fat, as opposed to muscle, bone, organs and water. Two people at 80 kg can look completely different at 12% and 25% — the scale cannot tell you that, but a body fat estimate can.
It matters because the goal of most training is not simply “lighter”. Losing fat while holding muscle is what changes a physique, and the percentage shows which of the two is actually moving.
Which formula do we use?
The US Navy method, developed by Hodgdon and Beckett in the 1980s, estimates body fat from circumference measurements. All input is converted to centimetres:
Men: %BF = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76
Women: %BF = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
The calculator then multiplies your body weight by the percentage to show lean body mass — everything that is not fat — and fat mass separately. Both numbers are useful: while cutting you want fat mass falling and lean mass flat.
How to use your result
Measure once a week, in the morning, before eating, after the bathroom, using the same tape in the same places. Consistency matters more than precision: a 1% swing between two well-taken readings is noise, not fat gain.
Compare four-week trends rather than single readings. Water, carbs and training volume move the number day to day; only a sustained direction means anything.
Finally, treat the category as a range, not a verdict. Lifters carry more muscle than average, so BMI-style judgments will misread you while this formula tracks your actual progress.
What the categories mean
The result is compared against ACE-style adult ranges. For men, essential fat sits around 2–5%, athletes around 6–13%, fitness enthusiasts 14–17%, average 18–24% and above 25% is considered high. For women the same bands shift up by roughly eight points, because essential fat is higher.
Use the category as context rather than a target. A lifter in the “average” band who is strong, lean-looking and improving is in a better position than an underweight person in the “essential” band. The two numbers worth watching week to week are fat mass falling and lean mass holding steady — everything else is framing.