Body Fat Percentage — A Better Number Than the Scale
I've watched people get genuinely discouraged by the scale not moving during weeks when they were clearly making progress — clothes fitting better, more energy, visible muscle definition. What's often happening is body recomposition: losing fat while gaining muscle, with total weight staying roughly the same. The scale can't see that. Body fat percentage can, at least approximately, and tracking it over time tells a more complete story than weight alone.
The US Navy Method — How It Works
Developed for military fitness assessments, the US Navy method estimates body fat from circumference measurements rather than skinfold calipers or expensive equipment. It uses height plus neck and waist measurements for men, and height plus neck, waist, and hip measurements for women.
%BF = 495 / (1.0324 − 0.19077×log10(34−15) + 0.15456×log10(70)) − 450
≈ 15.6% body fat — within the "fitness" range
How Accurate Is It Really?
Studies comparing the US Navy method to DEXA scans (considered a gold-standard for body composition) typically find it's accurate within about 3-4 percentage points for most people — reasonably good for a measurement that requires nothing but a tape measure. It tends to be less accurate for people at the extremes — very lean athletes or people with very high body fat — and for people with unusual body fat distribution.
For tracking changes over time, the method's consistency matters more than its absolute accuracy. If you measure the same way each time, a trend from 22% to 19% body fat over three months is meaningful even if the absolute numbers carry some error.
Why Waist Circumference Specifically Matters
Waist circumference is one of the strongest non-invasive predictors of visceral fat — the fat stored around abdominal organs that's most strongly linked to metabolic health risks like type 2 diabetes and cardiovascular disease. This is part of why the Navy method, despite its simplicity, tends to perform reasonably well: waist circumference is doing a lot of the predictive work.
Independent of body fat percentage calculations, a waist circumference above 40 inches (102cm) for men or 35 inches (88cm) for women is associated with increased health risk according to most clinical guidelines — worth knowing on its own.
Essential Fat vs. Storage Fat
Not all body fat is the same. Essential fat — found in cell membranes, bone marrow, nerve tissue, and organs that cushion and protect — is required for basic physiological function. The minimum is roughly 2-5% for men and 10-13% for women (the higher minimum for women relates to hormonal function and reproduction). Storage fat is the energy reserve that varies based on diet and activity — this is what most people mean when discussing "losing body fat."
Going below essential fat levels — which sometimes happens with extreme dieting or certain eating disorders — causes serious health consequences including hormonal disruption, loss of menstrual cycle in women, and impaired immune function. The "essential fat" minimums aren't arbitrary; they represent a real physiological floor.
Body Fat vs. BMI — Which Should You Track?
BMI is faster to calculate (just height and weight) and useful for population screening. Body fat percentage requires more measurements but gives a more direct picture of body composition — useful for anyone strength training, since BMI can't distinguish muscle gain from fat gain. If you're making body composition changes through training and diet, body fat percentage trends are more informative. If you just want a quick general health check, BMI is faster and still useful.