SAC rate calculator — know your gas consumption
Every gas plan you will ever make rests on one number: how much gas you breathe per minute. DecoLog's gas lab calculates your SAC (surface air consumption) and RMV (respiratory minute volume) from any dive, and your logbook turns every logged dive into a consumption trend. Here's what the numbers mean and how to measure them properly.
SAC vs RMV — they are not the same thing
SAC is usually quoted as pressure per minute — bar/min or psi/min, normalised to the surface. It's easy to read straight off your gauge, but it is tied to the cylinder you measured it on: one bar drawn from a 24 L twinset is twice the gas of one bar from a 12 L single. Swap tanks and the number stops meaning anything.
RMV fixes that by converting consumption into an actual volume of gas per minute, expressed at surface pressure — litres/min (metric) or cubic feet/min (imperial). Because it's a true volume, RMV is a property of the diver, not the cylinder, and it transfers to any tank, any depth, any dive. Serious gas planning is always done in volume.
How to measure it
Pick a segment of a dive with a reasonably constant depth, and record four things: start pressure, end pressure, average depth, and time. Then:
- Gas used = pressure drop × cylinder water volume (a 50 bar drop on a 12 L cylinder ≈ 600 surface litres).
- Rate at depth = gas used ÷ time.
- RMV = rate ÷ average absolute pressure in atmospheres — this normalises the figure to the surface.
Absolute pressure in seawater is roughly 1 ata at the surface plus 1 ata per 10 m, so 20 m ≈ 3 ata and 30 m ≈ 4 ata.
Worked example — metric
You swim a reef at a steady 20 m (3 ata) for 20 minutes on a 12 L cylinder, going from 200 to 150 bar.
- Gas used: 50 bar × 12 L = 600 litres (surface equivalent).
- Rate at depth: 600 ÷ 20 min = 30 L/min of surface gas.
- RMV: 30 ÷ 3 ata = 10 L/min. (As pressure-SAC on this tank: 50 bar ÷ 20 min ÷ 3 ata ≈ 0.83 bar/min.)
Worked example — imperial
Same idea with an AL80 (77.4 ft³ at 3000 psi, so ≈ 0.0258 ft³ per psi). You use 500 psi in 20 minutes at 66 ft (3 ata):
- Gas used: 500 psi × 0.0258 = 12.9 ft³.
- RMV: 12.9 ÷ 20 min ÷ 3 ata ≈ 0.22 ft³/min.
The cylinder-specific conversion factor (rated volume ÷ rated pressure) is exactly why imperial divers must know their "tank factor" — and why volume-based planning is cleaner.
Why this feeds rock-bottom planning
Your reserve gas — rock bottom, or minimum gas — is sized for the worst case: an out-of-gas teammate at the deepest point, two stressed divers sharing one supply through a full ascent with stops. That calculation is RMV × ambient pressure × time, for two divers, at an elevated breathing rate — an emergency is precisely when consumption spikes, so never plan reserves from your best relaxed number. Get the honest baseline first, then add margin. DecoLog's decompression planner uses your RMV to compute the gas requirement of every segment of the plan, and the logbook derives SAC/RMV automatically from each dive's cylinder and pressures, so your planning number comes from data instead of optimism.
Measure it now
The SAC/RMV calculator in DecoLog's gas lab does the arithmetic above in both unit systems — enter tank, pressures, depth and time, and get your number. Free, offline, no signup.
⚠ DecoLog is a planning aid, not a substitute for a dive computer or proper training. Gas planning decisions are yours — verify every number independently and dive within your certification.