Scuba RMV Calculator and SAC Calculator

Use this scuba RMV calculator to estimate your Respiratory Minute Volume (RMV) and Surface Air Consumption (SAC) rate from a real dive. RMV is one of the most useful numbers in gas planning because it tells you how much gas you breathe per minute at the surface in cubic feet per minute. SAC gives you the same information in PSI per minute, but only for that specific tank.

If you want to compare breathing rates across different dives, tanks, or conditions, RMV is the better number. If you always use the same cylinder, SAC can still be useful for logging and quick reference. This tool calculates both.

RMV and SAC Formulas

SAC describes gas consumption at the surface in PSI per minute. It is easy to calculate, but it is tied to a specific tank and becomes less useful when you switch cylinders or configurations.

\[ SAC = \frac{\left(\frac{\text{Starting PSI} - \text{Ending PSI}}{\text{Dive Time in Minutes}}\right)}{\text{Average Depth in ATA}} \]

RMV converts that same gas use into cubic feet per minute at the surface. Because it is volume-based rather than pressure-based, it is more useful for comparing dives and planning future gas needs.

\[ RMV = \text{SAC} \times \left(\frac{\text{Tank Size in cuft}}{\text{Tank Rated Pressure in PSI}}\right) \]

RMV vs. SAC

SAC and RMV both describe gas consumption, but they are not equally useful. SAC is measured in PSI per minute, which makes it specific to a particular cylinder. RMV is measured in cubic feet per minute, which makes it tank-independent and much better for planning dives across different tanks and conditions.

If you want the full explanation, read RMV vs. SAC: What’s the Difference and Why It Matters.

What Your Result Means

Your RMV is your estimated surface gas consumption in cubic feet per minute. Lower numbers usually reflect calmer, easier dives. Higher numbers often reflect current, cold water, stress, task loading, poor trim, poor buoyancy, or simply a harder dive profile.

Your SAC rate tells the same story, but only in terms of PSI per minute for that specific tank. That makes it convenient for logbook use, but less useful for broader gas planning.

Do not assume one dive defines your normal rate. Use several representative dives and plan conservatively.

How Divers Actually Use RMV

RMV becomes useful when you start planning gas instead of just recording it. Once you know your RMV, you can estimate how much gas you will need at depth, compare your gas use across different dives, and build more realistic reserve and turn-pressure plans.

This is one reason experienced divers usually think in RMV rather than SAC. RMV works across different cylinders and configurations in a way SAC does not.

For a deeper look at how this fits into real dive planning, see Recreational Gas Planning.

Common RMV Calculation Mistakes

• Using a stressful or unusual dive as your baseline
• Guessing at average depth instead of using dive computer data
• Comparing SAC rates across different tank sizes
• Ignoring the tank's actual rated pressure
• Using an optimistic number instead of a conservative planning number

A useful RMV is not your best number. It is a realistic number you can trust when the dive is harder than expected.

Bottom Line

If you want a simple logbook reference, SAC is fine. If you want a number you can actually use for gas planning, RMV is the better tool. Calculate both, understand the difference, and use a conservative RMV when planning future dives.