Dive Planning

Rock Bottom Gas Calculator

Calculate the minimum gas a team needs to share one supply and ascend from the deepest point of a dive, then convert it to a reserve pressure for your cylinder.

Dive and team

The deepest point of the dive and how the team breathes under stress.

Maximum depth

Stressed RMV per diver

L/min

Divers sharing the gas

divers

Exit profile

These assumptions are the safety guidance. Use the values your training organization teaches for the environment you dive.

Time to solve the problem at depth

min

Ascent rate

m/min

Stop depth

Stop time

min

Cylinder

The gas volume is the same for any cylinder; the reserve pressure depends on the one each diver carries.

Cylinder water volume

Plan the dive above this reserve in the gas planner

Emergency floor only

Rock bottom is the gas that must still be in the cylinder when the ascent begins from the deepest point. It is not a turn pressure, does not include the planned dive, and cannot model decompression obligations, overhead environments, currents, equipment failures beyond a single shared supply, or every emergency. Apply the method and reserves taught by your training organization.

Segment breakdown

Solve the problem at depth
1 min
240 L
Ascend to the stop
2.8 min
458 L
Hold the stop
3 min
270 L
Ascend to the surface
0.6 min
42 L

About rock bottom

Rock bottom, or minimum gas, is the reserve a team needs for a gas-sharing emergency at the most demanding point of the dive. Every segment is charged at its ambient pressure, so the deepest minutes are the most expensive.

Formula:

V = n \, \text{RMV} \sum_{i} t_i \, \bar{P}_i

Where:

• $n$ = divers breathing from the reserve
• $t_i$ = minutes in each exit segment
• $\bar{P}_i$ = mean ambient pressure of the segment in ATA

Reserve pressure divides that volume by the cylinder's capacity and is rounded up to the next 5 bar so it can be read on a gauge.

Result

95 bar reserve

1010 L for the team (505 L per diver), rounded up from 91 bar.

Team gas: 1010 L
Exact pressure: 91 bar

Start over

Frequently Asked Questions

What is rock bottom gas?

Rock bottom, also called minimum gas, is the reserve a team must still have at the deepest point of a dive so that two divers can share one gas supply, solve the problem, and ascend with any planned stop. It is an emergency floor, not the gas for the planned dive.

Why is my rock bottom higher than 50 bar or 500 psi?

A fixed reserve ignores depth, cylinder size, team size, and stressed breathing. Two divers sharing gas from 30 m / 100 ft with a realistic stressed RMV need roughly 1,000 L / 34 cu ft, which is about 85 bar in a 12 L cylinder or 1,300 psi in an AL80. When the calculated value exceeds a fixed reserve, the calculated value controls.

What stressed RMV should I use?

Use a rate well above your relaxed consumption. Many training organizations plan on roughly double a calm rate; 30 L/min or 1.0 cu ft/min per diver is a common planning figure. Measure your own SAC and RMV on comparable dives and choose a conservative value.

Is rock bottom the same as turn pressure or the rule of thirds?

No. Rock bottom is the emergency reserve floor for a direct ascent. The rule of thirds allocates gas for an outbound leg, the return, and a reserve. Where a dive requires a return to an exit, apply both and let the more conservative result set the turn.

Why do the same inputs give different pressures in different cylinders?

The gas volume the team needs is independent of the cylinder. Pressure is that volume divided by the cylinder's capacity, so a smaller cylinder shows the same reserve at a higher pressure. Each diver should calculate the pressure for the cylinder they actually carry.

Related Guides

Gas planning & consumption

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.

Gas planning & consumption

Turn Pressure and the Rule of Thirds

Calculate a usable turn pressure, distinguish thirds from minimum gas, and choose the earlier limit for direct-ascent and return-to-exit dives.

Gas planning & consumption

Understanding SAC Rate: Why It Matters for Every Diver

Learn what Surface Air Consumption rate is, how to calculate it, and tips to improve your breathing efficiency underwater.

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