The US Navy method estimates body fat from a small number of circumference measurements rather than direct measurement of fat mass. It was developed as a practical field alternative to underwater weighing, which is accurate but requires specialised equipment.
The formulas differ by sex because fat distribution patterns differ systematically between men and women — the female formula includes hip circumference specifically to capture typical lower-body fat storage that the male formula does not need to account for.
Measurement technique matters more than the formula
Because the calculation is sensitive to small differences in the inputs, consistent measurement technique matters. Waist should be measured at the narrowest point, typically just above the navel, without pulling the tape tight enough to compress the skin. Neck should be measured just below the larynx. Small measurement errors of even a centimetre or two can shift the result by a percentage point or more.
Accuracy compared with other methods
| Method | Typical accuracy | Practicality |
|---|---|---|
| DEXA scan | Very high (±1-2%) | Requires clinical equipment |
| Hydrostatic weighing | High (±2-3%) | Requires specialised tank |
| Skinfold calipers | Moderate (±3-4%) | Requires training to use consistently |
| Navy circumference method | Moderate (±3-4%) | Just a tape measure |
| Bioelectrical impedance (home scales) | Variable, sensitive to hydration | Very convenient |
Why this is an estimate, not a diagnosis
Any circumference-based method assumes a fairly typical fat distribution pattern. It systematically over- or under-estimates for people whose build differs substantially from that assumption — very muscular individuals, for instance, often get overestimated body fat readings from circumference methods, because the formula cannot distinguish a thick neck or waist caused by muscle from one caused by fat. Treat the result as a tracking tool for change over time using a consistent method, rather than a precise absolute figure.