Dive Air Consumption (SAC/RMV) Calculator
Work out your breathing rate from a real dive, then plan gas time at any depth on any cylinder — with the reserve rules that make the plan usable.
Work out your rate from a real dive
Plan a dive with that rate
RMV or SAC?
Both describe how fast you breathe, normalised back to the surface. The difference is the unit. SAC in psi per minute is tied to one cylinder — the same breathing on a small tank empties it faster in pressure terms, so a SAC figure only compares against dives on the same bottle.
RMV in cubic feet per minute is a real volume, so it transfers. Work it out once on an aluminium 80 and you can plan a dive on a steel 120 with it. That is the figure worth writing in your logbook.
Both scale with depth, and that is the part new divers underestimate. At 33 feet you are breathing twice the surface volume for the same lungfuls; at 99 feet, four times. A tank that lasted an hour in the shallows lasts a quarter of that at depth, and no amount of relaxed breathing changes the physics.
If your rate is high
It is nearly always a diving problem rather than a lung problem, and usually the same one: over-weighting. Excess lead forces you to inflate the BCD, which adds drag, which pushes you out of trim, which means finning harder to hold depth — and finning hard is what burns gas. Fix the weighting and the consumption follows. After that, look at trim, being cold, and simply going slower.
How to use
- Enter the pressure used, depth and time from a real dive.
- Read your surface consumption rate.
- Plan gas time at a new depth and cylinder size.
- Plan your reserve before you plan your bottom time.
Frequently asked questions
What is a surface air consumption rate?
How much gas you breathe per minute at the surface, derived by taking your consumption on a real dive and correcting for the pressure at depth. It is personal and reasonably stable, which is what makes it useful for planning — an average figure from a table is not your figure.
Why does depth change consumption so much?
Because gas is delivered at ambient pressure, so at 10 metres you breathe twice the mass of gas per breath as at the surface, and at 30 metres four times. Depth is the dominant variable in gas planning, far more than duration, which is why deep dives are short.
What is the rule of thirds?
A gas reserve rule: one third out, one third back, one third held in reserve. It applies where you must return the way you came — in overhead environments particularly — and it is deliberately conservative. Open-water recreational diving more commonly uses a fixed reserve pressure instead.
What raises my consumption on a given dive?
Cold, exertion, current, stress, poor buoyancy control and an ill-fitting or unfamiliar configuration. New divers typically breathe considerably more than experienced ones doing the same dive, and consumption falls markedly with experience as workload and anxiety drop.
Can I plan a dive from this alone?
No. Gas is one part of a dive plan, and decompression obligation, no-stop limits, thermal exposure, the specific site and your training are the rest. Gas planning that ignores the decompression ceiling can produce a plan you cannot legally or safely execute.
Is this a substitute for training?
Absolutely not. Diving is taught by certified instructors, gas planning is part of that training, and this is a calculator for someone who already knows what to do with the number. Nothing here is dive training or a substitute for a plan agreed with your buddy.
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