The U.S. Navy method estimates body fat percentage from a few tape-measure circumferences rather than expensive scanning equipment. It's not as precise as a DEXA scan, but it's free, repeatable, and good enough to track trends over weeks and months.
For men: 495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450. For women, the formula also factors in hip measurement: 495 / (1.29579 - 0.35004 x log10(waist + hip - neck) + 0.22100 x log10(height)) - 450. All measurements are in centimetres, taken at the narrowest point for the neck and waist.