What are SAC and RMV?
SAC (surface air consumption) is how much cylinder pressure you use each minute, scaled to the surface. It is measured in bar per minute or psi per minute.
RMV (respiratory minute volume) is how much gas you breathe each minute, scaled to the surface. It is measured in litres per minute or cubic feet per minute. Because it is a volume, it does not depend on the cylinder you used.
Gas is compressed by the water around you, so the deeper you are the more you use for each breath. SAC and RMV remove the depth from the figure so you can compare dives to different depths.
How to work them out
Note your start and end pressures, your average depth and how long you spent at it. Your dive computer or your logbook will have these.
Absolute pressure in atmospheres is the average depth divided by 10 plus 1 in metres, or divided by 33 plus 1 in feet. SAC is the pressure used divided by the absolute pressure and the time in minutes.
For RMV, multiply the pressure used by the cylinder size to get the gas used, then divide by the absolute pressure and the time. In litres, RMV equals SAC multiplied by the cylinder water capacity.
For example: 200 bar down to 50 bar in a 12 litre cylinder at an average of 18 metres for 40 minutes. That is 150 bar used, so 1,800 litres of gas. At 2.8 atmospheres over 40 minutes, SAC is about 1.3 bar per minute and RMV is about 16.1 litres per minute.
What is a typical rate?
Many recreational divers breathe roughly 12 to 25 litres per minute (about 0.4 to 0.9 cubic feet per minute) while relaxed at depth. New divers are often higher and experienced divers often lower.
Your own figure matters more than an average. Work it out over several dives and use the higher figures when you plan.
What changes your consumption?
Effort, cold, stress, poor trim and buoyancy, a strong current, and new or heavy equipment all raise it. Experience, relaxed breathing and good buoyancy bring it down.
Rates measured while working hard or while stressed are higher than on an easy dive. Divers often plan with a higher rate than they usually see.
Assumptions and limits
The calculator uses one average depth for the whole time, so a dive with a long shallow part and a short deep part will not be described well. Use the average depth from your dive computer.
It treats 10 metres or 33 feet of water as one atmosphere, as in seawater, and treats the gas as ideal. It ignores how real gas behaves at very high pressures, so a real cylinder can differ slightly from its rated volume.
This page is information, not a dive plan. Always follow your training, your agency's rules and your dive computer, and plan your reserves with your buddy.