New divers often ask how long their tank will last underwater. For training centers and rental operators, answering this clearly means fewer accidents—and more confident divers.The 120 Rule is a simple estimation formula: subtract your planned depth (in feet) from 120, and the result is your expected dive time in minutes with a standard aluminum 80 tank. This isn’t a strict safety rule, but it gives students a fast way to estimate dive time and encourages proper air management habits. As a supplier, understanding and promoting rules like this helps you guide your buyers to choose the right tanks for