This is the goal standard for measuring body composition. All other methods are tested for accuracy against this technique. Sitting on a chair, in water, suspended from a scale that looks much like a produce scale you exhale completely, duck your head under the water and sit as still as possible for five seconds while the technician "weighs" you. The more you weigh, the more muscle you have. The lighter you are, the more fat you have.

Advantage: The most accurate way to measure body composition. Drawback: Not Widely Available.

Cost $25-$200

Where you can get it: University exercise physiology laboratory