It estimates body fat percentage from body circumference measurements — neck and waist for men; neck, waist, and hips for women — using a formula developed by the U.S. Navy in the 1980s.

Studies show accuracy of ±3–4% compared to the gold-standard DEXA scan. It is considerably less accurate than hydrostatic weighing but far more practical at home.

For men: athletic 6–13%, fit 14–17%, acceptable 18–24%, obese 25%+. For women: athletic 14–20%, fit 21–24%, acceptable 25–31%, obese 32%+. These are general guidelines; ranges vary by age.

For this formula, measure at the level of your navel (belly button). Keep the tape parallel to the floor, relaxed but not compressing the skin. Take the measurement after a normal exhale.