Dive Gas Planning Calculator (Rule of Thirds & Rock Bottom)

How it works

Gas = SAC × (depth/10 + 1) × time; bar = litres ÷ cylinder volume; rock bottom = 2 divers × stressed SAC × (1 min at depth + ascent at 9 m/min at average pressure + 3 min at 5 m); thirds: turn at 2/3 of the fill

Gas planning turns the abstract “watch your gauge” into numbers before you get wet. Consumption scales with absolute pressure — at 30 metres you breathe four times the surface volume per minute — so a surface rate of 18 L/min becomes 72 L/min, and a 12-litre cylinder at 200 bar lasts far less than beginners expect. The reserve is the critical part: rock-bottom gas is the amount two divers need to ascend together from maximum depth at a stressed breathing rate, including a safety stop, after one of them has lost all gas; you must begin your ascent before the gauge falls to it. In caves, wrecks and under ice the rule of thirds applies — a third out, a third back, a third for emergencies. Whatever remains is the gas actually available for the dive.

Worked example

A 12 L cylinder at 200 bar (2,400 L), SAC 18 L/min, 24 m for 30 min: bottom gas 1,836 L (153 bar). Rock bottom for two divers (stressed ascent plus a 3-minute stop) is about 826 L (70 bar), leaving 1,574 L — only 26 minutes of bottom time, so the 30-minute plan does not fit: shorten it or go shallower. On the rule of thirds you would turn at 133 bar.

Frequently asked questions