This calculator is for education and dive planning practice. It does not replace training, your agency’s gas planning rules, a dive computer or your buddy check. Always end dives with the reserve you planned.
SAC and RMV
Air consumption rises with depth because each breath at depth holds more gas: at 33 ft of seawater the pressure is 2 atmospheres, at 66 ft 3 atmospheres. To compare dives, divers convert consumption back to the surface:
- SAC (surface air consumption), in psi per minute: pressure used ÷ minutes ÷ (depth ÷ 33 + 1).
- RMV (respiratory minute volume), in cubic feet per minute: SAC × tank capacity ÷ rated pressure. RMV does not depend on the tank, so it is the better number to remember.
Example
You used 900 psi in 20 minutes at an average 50 ft on an aluminum 80 (77.4 cu ft at 3,000 psi):
- SAC = 900 ÷ 20 ÷ 2.52 = 17.9 psi/min
- RMV = 17.9 × 77.4 ÷ 3,000 = 0.46 cu ft/min
Planning a dive
At a planned 66 ft, ambient pressure is 3 atm, so you use 17.9 × 3 = 53.7 psi/min. Starting at 3,000 psi and keeping 500 psi in reserve leaves 2,500 psi, about 47 minutes. This ignores the ascent, the safety stop, stress and cold, all of which raise consumption. Many divers plan with the rule of thirds (one third out, one third back, one third reserve) or a calculated “rock bottom” reserve for two divers sharing air to the surface.
Typical values
| RMV | Profile |
|---|---|
| Under 0.5 cu ft/min | Calm, experienced diver |
| 0.5 to 0.8 | Typical recreational diver |
| Over 0.8 | New, cold, working hard or stressed |
How to measure yours
Note the pressure at the start and end of a steady part of a dive, at a constant depth, and the time spent there. Repeat over several dives and use a conservative (higher) value for planning.
Lowering your consumption
Good buoyancy and trim, slow fin kicks, a well-fitted exposure suit and relaxed, deep breathing all help. Fitness matters too. Never skip breaths to save air; it builds up carbon dioxide.