SCUBA Air Consumption Calculator
Depth where you spent most of the dive
Total duration of the dive
Standard Aluminum 80 is 80 cu ft
Rated pressure for the tank volume
Gas Consumed:
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Ambient Pressure (ATA):
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SAC Rate (Pressure):
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RMV (Volume):
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*SAC (Surface Air Consumption) is measured in psi/min. RMV (Respiratory Minute Volume) is measured in cubic feet/min.
How to Calculate Air Consumption Rate
Understanding your air consumption rate is one of the most critical skills for a SCUBA diver. It allows you to plan dives safely, ensuring you have enough gas to complete your objective and return to the surface with a safety reserve. This calculator helps you determine two key metrics: SAC Rate (Surface Air Consumption) and RMV (Respiratory Minute Volume).
Understanding SAC vs. RMV
While often used interchangeably, these terms represent the gas usage in different units:
- SAC Rate (psi/minute): This tells you how much pressure (psi) you breathe from a specific tank per minute, adjusted to surface pressure. Note that because this relies on PSI, it is specific to the size of the tank you used. A SAC rate of 20 psi/min on an Aluminum 80 is very different from 20 psi/min on a Steel 100.
- RMV (cubic feet/minute): This converts your pressure usage into actual volume. This is the "gold standard" because it is independent of the tank size. An RMV of 0.5 cu ft/min applies whether you are using a single tank, double tanks, or a pony bottle.
The Calculation Formulas
To calculate these rates manually, we use physics based on Boyle's Law. Here is the step-by-step logic used in the calculator above:
1. Determine Gas Consumed
First, we find out how much gas was actually used during the dive:
Gas Used (psi) = Start Pressure - End Pressure
2. Calculate Average Depth in Atmospheres (ATA)
We must normalize the consumption to surface pressure. In imperial units (feet), atmospheric pressure increases by 1 ATA every 33 feet.
ATA = (Average Depth / 33) + 1
3. Calculate SAC Rate
We divide the total gas used by the time, and then divide again by the pressure (ATA) to normalize it to the surface.
SAC (psi/min) = (Gas Used / Time) / ATA
4. Calculate RMV
To convert pressure (SAC) to volume (RMV), we need the tank factor. The tank factor is the rated capacity divided by the rated working pressure.
Tank Factor = Rated Capacity / Working PressureRMV (cu ft/min) = SAC × Tank Factor
Why Do These Numbers Matter?
Knowing your RMV allows you to:
- Plan Deep Dives: At 99 feet (4 ATA), you will consume air 4 times faster than at the surface. If your RMV is 0.5 cu ft/min, at 99 feet you are breathing 2.0 cu ft/min.
- Select the Right Tank: If you know your consumption rate is high, you can choose a larger steel tank (e.g., 100 or 120 cu ft) instead of a standard Aluminum 80 to ensure you have ample bottom time.
- Monitor Improvements: As you gain experience, streamline your gear, and improve your buoyancy, your air consumption rate typically drops, allowing for longer dives.
Typical Air Consumption Rates
While every diver is different, here are general benchmarks for RMV:
- New Diver / High Stress: 0.75 – 1.0 cu ft/min
- Average Diver: 0.50 – 0.70 cu ft/min
- Experienced / Relaxed Diver: 0.35 – 0.50 cu ft/min