Archimedes' Principle

The principle that an immersed object is buoyed up by a force equal to the weight of the fluid it displaces, the physics behind all diver buoyancy.

Archimedes' principle states that any object in a fluid is pushed upward by a force equal to the weight of the fluid it displaces. A diver plus gear displaces a certain volume of water. If that displaced water weighs more than the diver, the diver floats; if less, the diver sinks; if the two are equal, the diver hangs motionless, neutrally buoyant, the state scuba divers spend their training learning to hold.

Everything about weighting flows from this principle. A wetsuit adds volume with little weight, so it floats you, and lead is added to compensate. Inflating the BCD increases displaced volume without adding weight, creating lift. Salt water is denser than fresh, about 1.03 kg per liter versus 1.00, so the same displaced volume weighs more and you need roughly 2 to 3 percent more lead in the ocean than in a lake.

Even your lungs are a buoyancy device: a full breath adds several liters of displacement, which is why fine control at a safety stop comes partly from breathing. A proper weight check is simply Archimedes' principle performed at the surface.