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 a fixed pressure reserve always enough?

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. Global Underwater Explorers, which renamed the reserve "minimum gas" in 2005, defines it as enough gas to surface from maximum depth or distance while sharing with an out-of-gas diver. The planning scenario therefore assumes one diver has lost access to their gas and both divers breathe from one supply while resolving the problem and ascending. The stressed RMV entered in the calculator is per diver; the calculation multiplies it by the number of divers.

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. Each segment's gas is its duration multiplied by the ambient pressure during it, and the segments are added:

V_{\text{min}} = n \times \text{RMV}_{\text{stressed}} \times \sum_{i} t_i \, \bar{P}_i

V_min is the team's minimum gas as a surface volume, n the number of divers sharing one supply, RMV_stressed each diver's stressed surface breathing rate in L/mincu ft/min, t_i a segment's duration in minutes, and P̄_i its average absolute pressure in bar or atmospheres. Ascent segments use the mean of their start and end pressures. Depth is converted to pressure with the sea-level seawater approximation of 10 m per bar33 ft per atmosphere.

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.

The calculator, figure, and table use one fixed exit profile: 30 L/min1.06 cu ft/min stressed RMV per diver, one minute to solve the problem at depth, an ascent at 9 m/min30 ft/min, and three minutes at 5 m15 ft. That profile is a DiveCalc teaching convention, not an agency standard, and it is less conservative than some methods. BSAC notes that a stressed diver can breathe 50 L/minabout 1.8 cu ft/min or more at the start of an incident, and Global Underwater Explorers' bailout calculation assumes a slower 3 m/min10 ft/min ascent to add margin. A higher breathing rate or a slower ascent raises the reserve. Use the rate, problem-solving time, and ascent the method you were trained in specifies.

A segment-by-segment example

For two divers at 30 m (4 bar absolute)99 ft (4 atmospheres absolute), using the profile's stressed rate of 30 L/min each1.06 cu ft/min each, the exit consumes:

Problem at depth: 240 L8.5 cu ft.
Ascent to the stop: 458 L16.2 cu ft, using average pressure during the ascent.
Three-minute stop: 270 L9.3 cu ft.
Final ascent: 42 L1.3 cu ft.

The total is about 1,010 L35.2 cu ft. Divided by water volume, that is 84.2 bar in a 12 L cylinder, rounded up to 85 bar, and 91.0 bar in an 11.1 L aluminum 80, rounded up to 95 bar.Divided by rated capacity and multiplied by working pressure, that is about 1,365 psi in an AL80 (77.4 cu ft at 3,000 psi), rounded up to 1,400 psi, and about 1,209 psi in a 12 L steel cylinder (98 cu ft at 3,365 psi), rounded up to 1,250 psi. Each is well above the commonly taught 50 bar500 psi fixed reserve, so the calculated minimum controls. The fixed reserve is a training convention, not a conversion: 50 bar is about 725 psi.

Rounding goes up, never down, because a higher reserve pressure ends the dive earlier. Use the water volume stamped on the cylinder, not a rounded size.Use the AL80's rated 77.4 cu ft rather than its nominal 80: dividing by 80 would understate the reserve by about 50 psi in this example.

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, with ambient pressure from the sea-level seawater approximation of 10 m per bar. This is DiveCalc's teaching profile, not an agency standard. 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, with ambient pressure from the sea-level seawater approximation of 33 ft per atmosphere. This is DiveCalc's teaching profile, not an agency standard. Cylinders use their rated capacity at working pressure. Rounded up to the next 50 psi.

Max depth	11.1 L aluminumAL80 (77.4 cu ft)	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 bar1150 psi	70 bar1000 psi	60 bar800 psi
30 m99 ft	95 bar1400 psi	85 bar1250 psi	70 bar1000 psi
40 m132 ft	125 bar1900 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 fixed 50 bar500 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. The standalone rock bottom gas calculator lets you change the team size, problem-solving time, ascent rate, and stop as well, and shows the gas each exit segment consumes. 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

GUE Procedural Changes, August 2005: Gas Management – Minimum Gas Nomenclature · Global Underwater Explorers — Primary agency definition: divers keep enough gas to surface from maximum depth or distance while sharing with an out-of-gas diver, renamed from rock bottom to minimum gas, with a caution that precise gas recommendations depend on many variables.
The Thought Process Behind GUE's CCR Configuration · InDEPTH Magazine — Defines Minimum Gas as the gas two divers need to ascend safely to the first gas switch, and shows GUE's bailout calculation using a 3 m / 10 ft per minute ascent for added conservatism; calls such reserves estimates.
Safe Diving Guide: Gas · British Sub-Aqua Club — Says reserves depend on cylinder capacity, dive plan, and both buddies' breathing rates; a stressed diver can breathe 50 L/min or more at the start of an incident; and a reserve should not be used to extend the dive.
Estimating Your Air Consumption · Divers Alert Network
Luxfer Aluminum Cylinder Specifications · Luxfer / XS Scuba — Source of the aluminum water volume, true capacity, weight, and seawater buoyancy figures (with valve, near-empty state at 500 psi / 35 bar).

Always use current training materials and operational procedures for dive planning.