Gas Consumption and SAC Rate
Your SAC rate tells you how fast you use gas. Learn how to calculate it, why it matters for dive planning, and how to improve it.
How long will your air last? Every diver asks this question, and the answer depends on your Surface Air Consumption rate. SAC rate is the single most important number for gas planning.
What it is
Surface Air Consumption (SAC) rate measures how much gas you consume per minute, normalized to surface pressure. It is expressed in bar per minute (bar/min) from your tank, or more precisely in liters per minute (L/min) of gas at surface pressure.
By normalizing to surface pressure, SAC lets you predict gas consumption at any depth — because your actual consumption rate at depth increases proportionally with ambient pressure (Boyle’s Law).
The key formulas
SAC rate (bar/min)
RMV (Respiratory Minute Volume, L/min)
| Variable | Meaning |
|---|---|
SAC | Surface Air Consumption in bar per minute |
RMV | Respiratory Minute Volume in liters per minute at surface pressure |
Gas used | Difference between starting and ending tank pressure (bar) |
Time | Duration at depth (minutes) |
RMV is more useful than SAC because it is independent of tank size. Your lungs do not care if you are breathing from an 11-liter or 15-liter cylinder.
Worked example
You spent 30 minutes at 20 meters and used 120 bar from an 11-liter tank. What is your SAC rate and RMV?
Step by step
Your SAC rate is 1.33 bar/min and your RMV is 14.7 L/min. This is in the typical range for a relaxed recreational diver.
Typical SAC rates
| Diver profile | RMV (L/min) | SAC with 11L tank |
|---|---|---|
| Experienced, relaxed | 10-14 | 0.9-1.3 bar/min |
| Average recreational | 14-18 | 1.3-1.6 bar/min |
| New diver or high workload | 18-25 | 1.6-2.3 bar/min |
| Stressed or heavy exertion | 25-40+ | 2.3-3.6+ bar/min |
These numbers vary significantly with fitness, experience, thermal protection, and psychological state. Your SAC rate on a calm reef dive is very different from your SAC rate fighting a current.
Using SAC for gas planning
Once you know your SAC rate, you can predict how much gas you need for any dive:
Example: planning a dive to 30 meters for 25 minutes
With SAC = 1.3 bar/min, 11L tank
Starting with 200 bar, this dive is tight. You would need to shorten bottom time or use a larger tank.
Why SAC rate matters
Turn pressure and gas planning
SAC rate determines when you need to turn a dive. For technical diving, gas management rules use SAC rate directly to calculate turn pressures and minimum gas requirements.
Buddy matching
Divers with very different SAC rates create a planning challenge. The dive is limited by the diver who consumes gas fastest. Knowing both SAC rates lets you plan accurately.
Equipment choices
Your SAC rate helps determine appropriate tank size. A diver with an RMV of 18 L/min needs a larger cylinder for the same dive plan than a diver with an RMV of 12 L/min.
How to improve your SAC rate
- Improve buoyancy control: Constantly adjusting buoyancy means constant fin movement, which burns gas
- Slow down: Swimming fast increases gas consumption disproportionately. Most divers swim too fast
- Breathe deeply and slowly: Long, full breaths exchange gas more efficiently than short, shallow breaths
- Streamline your gear: Dangling equipment creates drag, requiring more effort to swim
- Stay warm: Shivering massively increases gas consumption
- Relax: Anxiety increases breathing rate. Experience and familiarity breed calm
- Improve fitness: Cardiovascular fitness directly correlates with breathing efficiency
Measuring your SAC rate
To get an accurate SAC rate:
- Dive at a constant depth for at least 10-15 minutes (longer is more accurate)
- Note your starting and ending pressure and exact time at depth
- Swim at a normal, relaxed pace — not resting, not sprinting
- Calculate using the formula above
- Average multiple dives — one measurement is not enough. Your SAC rate varies with conditions
Most divers should measure their SAC rate periodically — it improves with experience and changes with fitness, equipment, and conditions.
Safety considerations
- Always plan with a stress SAC rate: For emergency planning (out-of-air scenarios, buddy breathing), use 2-3 times your normal SAC rate. Stress dramatically increases gas consumption
- Carry a reserve: Never plan to use your entire tank. A 50-bar reserve is the minimum for recreational diving
- Monitor throughout the dive: Check your pressure gauge frequently. A sudden increase in consumption may indicate a leak or increased stress
- Nitrox does not change SAC: Your breathing volume is the same regardless of gas mix. Nitrox gives you more time because of reduced nitrogen loading, not because of reduced gas usage
Sources
- NOAA Diving Manual, 6th Edition
- PADI Open Water Diver Manual
- Mount, T. & Dituri, J. (2008). “Exploration and Mixed Gas Diving Encyclopedia”