What is rock bottom gas in scuba diving?

Rock bottom, or minimum gas, is the calculated reserve needed for a team to manage a gas-sharing emergency and ascend from the most demanding point of the dive.

Is 50 bar or 500 psi always enough reserve?

No. A fixed reserve does not account for depth, ascent time, cylinder size, team size, or elevated breathing rates. Calculate minimum gas for the actual dive and follow your training.

Is rock bottom the same as the rule of thirds?

No. Rock bottom establishes an emergency reserve floor. The rule of thirds divides available gas for the outbound leg, return, and reserve. Some dives require comparing both approaches.

Diving guide

Rock Bottom Gas: How Much Reserve Do You Really Need?

Calculate minimum gas for a two-diver emergency ascent and learn how depth, breathing rate, and cylinder size change reserve pressure.

A fixed reserve is easy to remember, but it cannot know how deep you are, how large your cylinders are, or how much gas two stressed divers need to reach the surface. Rock bottom—also called minimum gas—turns those variables into a reserve designed for the dive.

What rock bottom means

Rock bottom is the gas that must remain available at the point where an emergency ascent would demand the most gas. A common planning scenario assumes one diver has lost access to their gas and both divers must breathe from one supply while resolving the problem and ascending. The stressed RMV entered in the calculator is per diver, the engine multiplies that rate by two.

Because gas consumption rises with ambient pressure, the minutes spent deepest are the most expensive. The calculation therefore breaks the exit into segments: problem-solving at maximum depth, ascent, any planned stop, and the final ascent.

The emergency exit that rock bottom protects

This profile mirrors the calculator's assumptions: two stressed gas-sharing divers, one minute to solve the problem, a controlled ascent, and a three-minute shallow stop. The deepest segment consumes gas fastest.

Try it: Rock Bottom Gas

Two divers, one minute resolving the problem, a controlled ascent, and a three-minute safety stop.

Maximum depth

Stressed RMV per diver

L/min

Cylinder water volume

1010 L

minimum gas for the team

85 bar

rounded reserve pressure per matching cylinder

Educational cross-check only. Use the assumptions and procedures taught by your training organization, and account for the actual team and dive environment.

Inputs that change the answer

Maximum depth: deeper water increases ambient pressure and consumption.
Stressed RMV: emergency breathing should not use a calm-diving rate.
Team size: a shared-gas ascent must supply everyone breathing from the reserve.
Cylinder capacity: the same gas volume corresponds to different pressure in different cylinders.
Exit profile: ascent rate, stops, overheads, and decompression obligations all matter.

A segment-by-segment example

For two divers at 30 m100 ft, using a stressed rate of 30 L/min each1.0 cu ft/min each, the example profile consumes:

Problem at depth: 240 L8.1 cu ft.
Ascent to the stop: 458 L15.5 cu ft, using average pressure during the ascent.
Three-minute stop: 270 L8.7 cu ft.
Final ascent: 42 L1.2 cu ft.

The total is about 1,010 L33.6 cu ft. That corresponds to 84 bar in one 12 L cylinder, rounded up to 85 bar, or about 1,258 psi in an AL80, rounded up to 1,300 psi. Both are larger than the familiar 50 bar / 700 psi fixed reserve, so the calculated minimum controls.

Reserve pressure for a two-diver ascent

Assumes two divers sharing gas at a stressed 30 L/min each, one minute to resolve the problem at depth, a 9 m/min ascent, and a three-minute stop at 5 m. Rounded up to the next 5 bar.Assumes two divers sharing gas at a stressed 1.06 cu ft/min each, one minute to resolve the problem at depth, a 30 ft/min ascent, and a three-minute stop at 15 ft. Rounded up to the next 50 psi.

Max depth	11.1 L aluminum80 cu ft aluminum	12 L steel98 cu ft steel	15 L steel123 cu ft steel
18 m59.4 ft	60 bar900 psi	60 bar800 psi	45 bar650 psi
24 m79.2 ft	75 bar1100 psi	70 bar1000 psi	60 bar800 psi
30 m99 ft	95 bar1350 psi	85 bar1250 psi	70 bar1000 psi
40 m132 ft	125 bar1800 psi	115 bar1650 psi	95 bar1350 psi

Read these as an illustration of how reserve scales with depth and cylinder size, not as pressures to dive. The assumed breathing rate, problem-solving time, ascent rate, and stop are planning choices your training organization sets, and a real dive may add overhead environments, decompression obligations, or a larger team.

Rock bottom versus turn pressure

Minimum gas is a floor, not automatically the pressure at which every dive turns. For a direct-ascent recreational dive, the team plans its usable gas above that floor. A dive requiring a return to an exit may apply an additional gas-allocation rule. Whichever method produces the earlier turn controls the plan. Likewise, when calculated minimum gas is larger than a conventional 50 bar / 700 psi reserve, the calculated value overrides the shortcut.

Use the result conservatively

Round reserve pressure up, agree on it with the team, and monitor the diver who will reach it first. Recalculate when the depth, cylinders, team, or exit changes. Use our gas consumption planner for the planned portion of the dive and the SAC guide to establish a realistic breathing rate.

Training matters: this educational estimate cannot model every emergency, environment, equipment configuration, or decompression obligation. Apply the method taught by your instructor or training organization.

Sources

References and further reading

These official and specialist sources support the concepts in this guide. Always use current training materials and operational procedures for dive planning.