Most recreational divers plan gas by watching the needle and turning around when it feels low. That works until the dive where it does not, and it fails at exactly the moment you have the least capacity to think.

The question every method is answering

How much gas do two divers need, starting from the worst point of the dive, to solve a problem and get to the surface at a safe ascent rate with a safety stop? Answer that once and the rest is arithmetic.

Three methods, increasing honesty

• Rule of thirds. A third in, a third out, a third in reserve. Simple, memorable, and designed for overhead environments where you must exit the way you came. It says nothing about depth, which is its weakness — a third of your gas at forty metres is not a third of your ability to leave.
• Minimum gas. Reserve a fixed pressure, often 50 bar. Easy, and wrong in opposite directions at different depths: far too much on a shallow reef, nowhere near enough deep.
• Rock bottom. Compute it. Two divers sharing one regulator, at the deepest point, breathing at an elevated rate because something just went wrong, plus the ascent, plus the stop. It gives you a number that changes with the dive, which is the point.

Why the third one wins

Because it is the only one that models the failure you are reserving gas for. The others reserve a quantity; rock bottom reserves a capability. The numbers are frequently uncomfortable — on deep dives the reserve can be more than half the cylinder, which tells you the dive plan was wrong, not that the method is.

The panels below work through a forty-metre example and the same profile at eighteen, because the difference between them is the whole argument.