Calculator
SAC rate calculator — Surface Air Consumption
Your SAC rate is the volume of gas (in litres at the surface) you breathe per minute. It's the basis of every gas plan — gas consumption at depth scales linearly with ambient pressure, so an accurate SAC matters most on long, deep dives. Log a dive's cylinder size, start and end pressure, average depth, and duration, and the calculator returns SAC.
How SAC is calculated
Gas consumed (in surface litres) is cylinder size times pressure drop. SAC normalizes that by ambient pressure and dive time — so a faster breather at depth produces a higher SAC, and the value works as a personal constant across dives.
SAC (L/min) = ((P_start − P_end) × V_cyl) ÷ (t × P_avg) V_cyl cylinder water volume (L) P_start starting pressure (bar) P_end ending pressure (bar) P_avg average ambient pressure during the dive (bar) t dive duration (min)
Worked example: 12 L cylinder, 200 → 100 bar, 20 m average depth (P ≈ 3.0 bar), 30 minutes. Gas used ≈ 1200 L at the surface. SAC = 1200 / (30 × 3.0) ≈ 13.3 L/min — a recreational-diver typical value.
How to use SAC
Run several relaxed dives to find your baseline SAC. Plan with a slightly higher number (10–20% margin) for harder dives — current, cold, anxiety, and workload all push it up. Many planners ask for SAC and an out-of-air SAC (often 2x) so they can size bailout and rock-bottom reserves.
Frequently asked questions
- What's a normal SAC rate?
- Recreational divers typically come in around 15–25 L/min. Experienced technical divers can drop into the 10–15 L/min range. Rebreather divers are not directly comparable — the metric there is metabolic O₂ consumption, around 1 L/min.
- Does SAC stay constant across dives?
- Within a few percent for the same diver in similar conditions. Add 20% if you're cold, working hard, or anxious. SAC drops as comfort and trim improve.
- What's the difference between SAC and RMV?
- RMV (Respiratory Minute Volume) is the lung-level metric — how much air you actually breathe per minute. SAC is RMV expressed in surface volume terms by dividing by ambient pressure. The numbers are equivalent at the surface; SAC is more useful for gas planning.
- How accurate does the cylinder size need to be?
- Errors compound linearly. The calculator uses a real-gas EOS, which corrects for compressibility at high pressures (steel 232 bar fills are off by several percent under the ideal-gas approximation). Use the cylinder's nominal water volume — most are stamped on the neck.