What is the 1 3 rule in diving?

In scuba diving, the rule of thirds is a rule of thumb used by divers to plan dives so they have enough breathing gas remaining in their diving cylinder at the end of the dive to be able to complete the dive safely. 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 remain underwater to do decompression stops before being able to ascend safely to the surface. A diver without gas cannot do the stops and risks decompression sickness. By the rule of thirds system the gas in stage cylinders is managed in the same way as the primary supply, whether the primary is carried as back gas or sidemounted. If all goes to plan, the divers will surface with stages and primary cylinders each containing about one third of the original content. With the rule of thirds, the duration of the dive is limited by the point at which the gas reaches 1/3 the starting quantity, by not exceeding the planned decompression obligation, and by returning along the same route in similar conditions.

The 1/3 Rule is a method of managing your air supply during a dive. One-third of your air is for the descent and exploration. One-third of your air is for the return to the surface. One-third of your air is reserved for emergencies. This safety buffer is crucial because it accounts for unexpected situations, such as stronger currents, slower ascents, or assisting a dive buddy in need. The 1/3 Rule prevents scuba divers from running out of air, one of the most dangerous scenarios. It also encourages good dive planning and situational awareness. Beyond personal safety, the 1/3 Rule is essential for maintaining your buddy system. If a buddy has an issue with their air supply, your emergency reserve can help both of you return to the surface safely.