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A tape measure beats a $50 smart scale.
The US Navy body fat formula estimates body composition from circumference measurements — no calipers, no scanners, no bioimpedance scale guessing at your hydration. Developed for military fitness assessments, it is the most validated no-equipment method available and typically lands within 3–4% of a DEXA scan. This calculator also maps your result to the American Council on Exercise categories.
People use this when the scale alone stops telling the truth — weight can drop while muscle goes with it. It is the no-equipment standard (developed for the US Navy), used before military fitness assessments, by trainers tracking clients, and by anyone who wants a trend line smarter than pounds.
Studies put it within roughly ±3–4% of DEXA for most people — better than consumer smart scales, which swing wildly with hydration. Consistency matters more than absolute precision: measure the same way, same time, weekly.
ACE guidelines: men 14–24% is the fitness-to-average range, women 21–31%. Essential fat (the floor for health) is roughly 2–5% for men and 10–13% for women. Athletes sit between the two.
Neck circumference corrects for frame size: a thick-necked 38-inch waist carries very different fat than a slim-necked one. The waist-to-neck difference is the core of the male formula.