Body Fat Calculator.

The US Navy formula estimates body fat percentage from circumference measurements — neck and waist for men, plus hip for women — combined with height. It was developed and validated by Hodgdon and Beckett at the Naval Health Research Center in 1984 as a field-deployable alternative to hydrostatic weighing for screening service members. It is a regression equation, not a direct measurement.

Against the laboratory gold standards — DEXA scan, hydrostatic weighing, BodPod — the US Navy method typically lands within ±3–4% for adults with average body composition. It tends to overestimate fat in very muscular individuals (large necks) and underestimate it in adults carrying weight predominantly in the limbs rather than the trunk.

BMI cannot separate muscle from fat. Two adults at the same height and weight — one lean and muscular, one sedentary — get identical BMI scores but radically different metabolic risk. The US Navy method side-steps that confound by sampling where fat actually accumulates: the waist (visceral and subcutaneous abdominal fat) and, for women, the hips (gynoid fat).

Body fat percentage also gives you a lever BMI cannot: lean body mass. Knowing your fat-free mass lets you set protein targets, refine TDEE estimates, and judge whether a weight change came from fat or muscle. It is the single most useful number to track over a cut or bulk.

Reference ranges from the American Council on Exercise: essential fat is 10–13% (women) / 2–5% (men); athletes 14–20% / 6–13%; fitness 21–24% / 14–17%; average 25–31% / 18–24%; obese 32%+ / 25%+. Risk for cardiometabolic disease climbs sharply above the average band and the gain comes mostly from visceral, not subcutaneous, fat.

Track the trend, not the absolute number. The error bars on any field method are wide enough that a single reading is noisy, but month-over-month change at the same measurement protocol (same tape, same time of day, same hydration state) is a reliable signal.

Three populations where the Navy formula breaks: bodybuilders or strength athletes with disproportionately large neck circumferences (overestimates fat by 3–6%); adults with central obesity but minimal hip/limb fat (overestimates); and adults who carry fat predominantly in extremities rather than the trunk (underestimates). If you fall in any of these groups, a DEXA scan ($75–$150) is worth the one-time spend for a baseline.