Scuba Rock Bottom Gas Calculator
Enter the depth, stressed breathing rates, ascent plan, and cylinder size to estimate the minimum gas reserve for a shared-gas ascent.
Rounded minimum pressure (bar)90
- Total minimum gas (L)
- 1,010
- Problem-solving gas (L)
- 240
- Ascent travel gas (L)
- 500
- Shallow-stop gas (L)
- 270
- Exact reserve pressure (bar)
- 84.17
- Pressure available above reserve (bar)
- 110
- Starting gas assessment
- Starting pressure clears the rounded reserve by 110 bar.
Gas by profile segment
| Segment | Time | Mean pressure | Gas (L) |
|---|---|---|---|
| Problem solving at depth | 1 | 4 | 240 |
| Ascent to shallow stop | 2.78 | 2.75 | 458.33 |
| Shallow stop | 3 | 1.5 | 270 |
| Ascent to surface | 0.56 | 1.25 | 41.67 |
Gas volume is surface-equivalent open-circuit gas for two divers sharing one supply.
The planning pressure is rounded upward to the next 10 bar.
This does not calculate decompression, overhead, staged-gas, or rebreather bailout requirements.
How to use this calculator
- Choose metric or imperial units before entering the dive profile.
- Enter each diver’s stressed RMV, not a relaxed surface breathing rate.
- Set the problem-solving time, ascent rate, and optional shallow stop.
- Enter the cylinder rating and starting pressure to compare the plan with the gas on hand.
How rock bottom gas is calculated
Rock bottom, also called minimum gas, is the gas pressure at which a team must leave the deepest point with enough gas for a controlled ascent while one diver is out of gas. This calculator treats the supply as one shared source and adds both divers’ stressed respiratory minute volumes.
The calculation uses the standard open-circuit gas formula:
gas = RMV × ambient pressure × time
Depth is converted to metres internally. Ambient pressure is approximated as A(d) = 1 + d ÷ 10 atmospheres absolute. If D is maximum depth, S is stop depth, v is ascent rate, tp is problem-solving time, ts is stop time, and R is the two divers’ combined stressed RMV, the problem-solving gas is:
Gp = R × A(D) × tp
Travel gas is integrated across each linear ascent segment by using the mean pressure over the segment:
G(x,y) = R × ((x − y) ÷ v) × (A(x) + A(y)) ÷ 2
With a stop, ascent gas is G(D,S) + G(S,0) and stop gas is R × A(S) × ts. With no stop time, the ascent is calculated directly from maximum depth to the surface. The total minimum gas is then converted to cylinder pressure: litres divided by water volume for metric cylinders, or cubic feet divided by rated capacity and multiplied by service pressure for imperial cylinders.
What moves the result most
Maximum depth and stressed RMV usually dominate the reserve. A small increase in depth raises both the pressure during the problem-solving minute and the pressure across the ascent. A slow ascent, longer stop, or smaller cylinder also raises the pressure reserve. Two divers with different breathing rates are handled directly; the calculator adds the donor’s and receiver’s entered RMVs.
What this leaves out
This is a direct open-water ascent model. It does not plan decompression, gas switches, penetration distance, stage bottle failures, lost buddy procedures, rebreather bailout, gas density, narcosis, thermal stress, current, or task loading. If the dive may require decompression or an overhead exit, use the gas-planning method taught for that environment and verify it with your team or instructor.
Worked example
Suppose two divers plan a metric ascent from 30 m. Each diver uses a stressed RMV of 30 L/min, so the combined RMV is 60 L/min. They allow 1 minute to solve the problem at depth, ascend at 9 m/min, stop at 5 m for 3 minutes, and share a 12 L cylinder.
Problem-solving gas is 60 × 4.0 × 1 = 240 L. Travel gas is 60 × (25 ÷ 9) × 2.75 = 458 L from 30 m to 5 m, plus 60 × (5 ÷ 9) × 1.25 = 42 L from 5 m to the surface. The stop adds 60 × 1.5 × 3 = 270 L. Total minimum gas is about 1,010 L, which is 84.2 bar in a 12 L cylinder, rounded up to a 90 bar planning reserve.
Common questions
What is rock bottom gas in scuba diving?
Rock bottom gas is the minimum gas a team keeps in reserve so two divers can share gas from the planned maximum depth and ascend in control. It is calculated from depth, time, ascent rate, stressed breathing rates, and cylinder size rather than a fixed pressure rule.
Why does the calculation include gas for two divers?
The usual rock bottom scenario assumes one diver has lost access to gas and must breathe from the donating diver’s supply. The reserve therefore has to cover both divers until they reach the surface or the planned exit point.
What stressed RMV should I enter?
Use a conservative breathing rate for a hard or stressful ascent, not your calm surface-equivalent rate from an easy dive. Many divers calculate RMV from past dives and then apply a stress multiplier chosen by their training agency, instructor, or team protocol.
Should rock bottom include a safety stop?
For no-decompression open-water dives, some teams include a short shallow stop and some omit it when planning emergency gas. Enter the stop your team intends to preserve during the gas-sharing ascent, or set stop time to zero for a direct ascent model.
How do I enter doubles or a manifolded twinset?
Enter the total supply available to both divers. For metric cylinders, use the combined water volume, such as two 12 L cylinders entered as 24 L. For imperial cylinders, use the combined rated capacity paired with the matching service pressure.
Is this the same as a 50 bar or 500 psi reserve?
No. A fixed reserve can be too high or too low depending on depth, breathing rates, cylinder size, ascent rate, and stop plan. This calculator converts a gas volume requirement into a cylinder-specific pressure reserve.