Most advanced divers skip it. Every technical diver lives by it. Respiratory Minute Volume, sometimes called SAC rate or Surface Air Consumption, is the volume of gas a diver breathes per minute, normalized to the surface. It’s one of the most important numbers in gas planning, and the majority of recreational divers have never calculated their own.

Why It Matters

Without an RMV rate, gas planning is a guess. With it, a diver can calculate exactly how much gas they’ll consume at any depth, for any planned bottom time, under any level of exertion, with any size tank.

How to Calculate It

Log starting and ending tank pressure, in PSI or BAR, from a dive. Note average depth and bottom time from a dive computer. Calculate gas consumed: starting PSI minus ending PSI, multiplied by tank volume in cubic feet, divided by the tank’s pressure rating. Divide that figure by bottom time to get consumption per minute. Then divide by the pressure factor at average depth, which is depth in feet divided by 33, plus 1.

The result is RMV rate in cubic feet per minute. Repeating this across ten to fifteen dives and averaging the results gives a reliable number. It’s worth tracking two versions: a relaxed RMV and a working RMV, since the two are typically very different.

A typical recreational diver runs between 0.6 and 0.8 cubic feet per minute at rest. Under stress or exertion, that figure can easily double.

From Reactive to Deliberate

Once a diver knows their number, gas planning stops being reactive and starts being deliberate. This is where mastery starts, not with the gear, but with the math behind it.