Rule of thirds (diving)

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In scuba diving, the rule of thirds is a rule of thumb that divers use to plan dives so they do not consume all the breathing gas from the diving cylinder before the end of the dive.

For divers following the rule, one third of the gas supply is planned for the outward journey, one third is for the return journey and one third is a safety reserve.

Reserves are needed at the end of dives in case the diver has gone deeper or longer than planned and must do decompression stops before being able to ascend safely to the surface.