Where the formula comes from
Hodgdon and Beckett (1984) built two regression equations from hydrostatically-measured body composition on Navy personnel — one for men using neck and waist, one for women using neck, waist and hip. The Navy adopted them as the official Body Composition Assessment tool because calipers are difficult to use reliably without training and DEXA scanners aren’t portable.
How accurate is “within ±3–4%” in practice?
If your true body fat is 18%, the tape estimate will land somewhere between 14% and 22% about 95% of the time. That’s good enough to track progress against yourself over months — improvement of 3% on the same body is a meaningful change — but not good enough to compare against a friend by 1 or 2 percentage points.
What this estimate doesn’t capture
Visceral vs subcutaneous distribution. Bone density. Lean-mass quality. Hydration status. The Navy method is designed for “is this person in combat-ready body composition?” — not for clinical body-composition tracking in elite athletes or people with metabolic disease. For those, ask for DEXA or BodPod.