Scuba Gas Planning Calculator
Know how much gas your dive needs, when you must start your ascent, and when to turn around.
Results update as you type.
Quick examples
Load a full example, then change any value.
Your normal breathing rate at the surface. You can work it out with the SAC calculator.
How long you plan to stay down.
The average depth of the planned dive.
Water capacity of the tank. Most rental tanks are 12 L.
Gas required
Within limitsThis dive needs about 2000 litres of gas. That's 170 bar in your 12 L tank, not counting any reserve.
- In your tank
- 170 bar
- Gas use at 15 m
- 50 L/min
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About gas planning
Gas planning answers three questions before a dive: how much gas the dive needs, how much has to stay in the tank for an emergency ascent, and when to turn around.
Required gas covers the planned dive only. Rock bottom is the reserve two divers need to share one tank on the way up. The rule of thirds splits the tank into the way out, the way back and a reserve.
With your numbers
Enter valid values to see the calculation worked through with your numbers.
Good to know
- Your tank has to hold the required gas plus the rock bottom reserve. If it doesn't, plan a shorter dive or take a bigger tank.
- Use a SAC from your own logged dives, and add a margin for cold water, current or a new site.
- Plan with the numbers of the diver who breathes more. The pair turns and ascends together.
- Keep at least 50 bar in the tank when you surface.
Formula
Required gas
G = SAC × t × Pavg
Pressure in the tank, rounded up to the next 5 bar
Ptank = (G) ÷ (V)
Rock bottom for two divers
GRB = 2 × SACs × (1 × Pmax + tasc × Pmid + 3 × P5 + tsurf × P2.5)
Turn pressure, rounded up to the next 5 bar
Pturn = Pfill − (Pfill) ÷ (3)
Where
- SAC, SACs: planning SAC and stressed SAC per diver (L/min)
- t: bottom time (min)
- Pavg, Pmax, P5: absolute pressure, d ÷ 10 + 1, at average depth, maximum depth and 5 m (bar)
- Pmid: absolute pressure halfway up, at (dmax + 5) ÷ 2
- tasc: ascent time to 5 m at 9 m/min, (dmax − 5) ÷ 9
- tsurf: ascent time from 5 m to the surface, 5 ÷ 9, at P2.5 = 1.25 bar
- 1 and 3: minutes at maximum depth and at the 5 m stop
- V: tank volume (L); Pfill: fill pressure (bar)
How it's calculated
- Required gas (L) = SAC × time × (average depth ÷ 10 + 1). Pressure = litres ÷ tank volume, rounded up to 5 bar.
- Rock bottom (L) = 2 divers × stressed SAC × (1 minute at maximum depth + the ascent to 5 m at 9 m/min, counted at its average depth + 3 minutes at 5 m + the last 5 m to the surface at 9 m/min, counted at 2.5 m), each minute weighted by absolute pressure. Pressure is rounded up to 5 bar.
- Rule of thirds: turn pressure = fill pressure − one third, rounded up to 5 bar.
These tools are educational planning aids; they are not a substitute for dive training, current printed tables or a dive computer. Verify results against your training and always plan within the limits of your certification.
How to use the gas planning calculator
- 1
Required Gas: enter your SAC rate, planned time, average depth and tank size to see how much gas the dive itself needs.
- 2
Rock Bottom: enter the maximum depth, a stressed SAC and your tank size to find the pressure at which you must start the ascent.
- 3
Rule of Thirds: enter your fill pressure to get the pressure at which you turn the dive around.
What the results mean
Required gas covers only the planned profile. Your tank has to hold this plus the rock bottom reserve.
Rock bottom is the gas two divers need to sort out a problem at the deepest point and ascend together while sharing one tank. Begin your ascent before your gauge drops to this pressure.
The rule of thirds is common in overhead environments and currents: one third going out, one third coming back, one third kept in reserve.