What is the 120 rule in scuba diving?

The 120 rule in scuba diving is a pre-dive-computer shortcut for staying inside no-decompression limits: depth in feet plus bottom time in minutes should add up to no more than 120. By the rule, a diver at 60 feet gets 60 minutes and a diver at 80 feet gets 40. Modern planning is stricter at those depths: PADI’s Recreational Dive Planner allows 56 minutes at 60 feet, and at 80 feet PADI puts the limit at about 25 minutes. So the 120 rule is a rough memory aid, and the dive computer’s no-decompression time is the one to follow. On BVI yacht charters, the dive is run either by a local dive operator that meets the yacht or by a crew member on yachts that carry their own gear.