This is taken from here.
USCG: PFD Selection, Use, Wear & Care of PFDs
These are the guys that create the specs for PFD's
- The tendancy of a body to float or sink in water or any other fluid. Most people will naturally float in water, especially if they fill their lungs with air. Most require only about 11 pounds (50 Newtons) of extra buoyancy to keep their head out of water. That is why a PFD with just 15.5 pounds (70 Newtons) of buoyancy can provide adequate flotation for an adult -- even a very large person. PFDs with 22 to 34 pounds (100 to 155 Newtons) can provide superior performance.
In technical terms, buoyancy is determined by Archimedes' Principle:
Any body partially or completely submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the body.
That means someone immersed in water is "buoyed" upward by a force equal to the weight of the volume of water that their body takes up (displaces). Gravity pulls a person's body downward by a force equal to their weight. The difference between these forces is a person's net buoyancy. A PFD is very light weight, but displaces enough water to make the PFD and the person wearing it very buoyant.
It also follows that the people hardest to float are those with compact, dense bodies. These tend to be people with athletic body builds, with a lot of bone and muscle mass, and not much fat. Fat is not as dense as muscle and bone, so people who are overweight can actually be easier to float than someone who is much smaller and leaner. Heavy people do not need a higher buoyancy PFD because of their weight.