Rule of Thirds

A gas planning rule from cave diving: one third of the supply for the way in, one third for the way out, and one third held in reserve.

The rule of thirds is a gas planning rule born in cave diving: use one third of your gas for the way in, turn the dive there, use the second third for the way out, and keep the final third as an untouched reserve. The reserve exists for one scenario above all, a teammate losing gas at the point of maximum penetration and needing to share yours for the entire exit.

The rule applies wherever a direct ascent to the surface is impossible, which is what makes overhead environments such as caves and wreck penetrations fundamentally different from open water. Serious overhead divers treat thirds as a starting point, not a guarantee: with mismatched cylinder sizes the team plans on the smallest gas supply, and factors like flow direction in a cave can demand more conservative fractions, such as sixths when swimming in with the current.

In open water the same logic appears in softer form as turn pressure planning, where the dive turns at a pre agreed gauge reading rather than a fixed fraction of supply. The underlying principle is identical everywhere: decide before the dive how much gas the return and the emergency deserve, commit to the number, and let the gauge, not enthusiasm, decide when the dive turns.