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. A diver at 80 feet, for example, gets 40 minutes of bottom time before nudging the deco limit; at 60 feet, 60 minutes. The rule is conservative — modern dive computers running RGBM or Bühlmann tables typically allow longer times. But on a Virgin Gorda day-charter where instructors brief without computers, the 120 rule still gets cited as a sanity check, especially for vacation divers who haven’t dived in months. Boat captains in the BVI use it the same way. Dive computers override it; the rule is a backup. BVI yacht charters commonly include certified dive crews.