The Navy method is a formula that estimates your body fat percentage from body circumferences and height. Hodgdon and Beckett developed it for the US Navy in 1984 and calibrated it against underwater weighing. All you need is a tape measure.
Formula and measuring points
For men, neck and waist go into the formula; for women, the hips as well. The original formulas work in inches (1 inch = 2.54 cm). For men:
Body fat (%) = 86.010 × log10(waist - neck) - 70.041 × log10(height) + 36.76
Measure your neck just below the larynx, your waist at navel height for men or at the narrowest point for women, and your hips at the widest point of the buttocks. The tape lies flat without digging in.
Example: A man who is 180 cm tall with an 86 cm waist and a 38 cm neck comes out at about 17 percent body fat. This is a worked example.
How accurate it is
The Navy method is an estimate. For groups it gives usable averages, but for an individual it can be off by several percentage points, for instance with a very thick neck or a lot of belly fat. A body fat scale often shows something different because it estimates on a different principle.
The trend is what matters most. Always measure the same way, in the morning before eating and with the same tape, and only every two to four weeks. For very small changes, such as during a recomp, the method is often too imprecise.