Surface Air Consumption (SAC) Rate Calculator
Calculate your Surface Air Consumption (SAC) rate from a dive log entry.
Use your SAC rate to predict tank duration for future dives.
What SAC actually measures
Surface Air Consumption (SAC) rate is how much gas you breathe per minute if you were standing at the surface — independent of depth. Since the same volume of air shrinks under pressure, you actually breathe more gas per minute at depth than at the surface. SAC factors that out, giving you a personal baseline that doesn’t move when your dive profile changes.
The formula:
SAC = (pressure_used × tank_volume) ÷ (average_pressure_ata × bottom_time_minutes)
Pressure used and tank volume give you total gas consumed in surface-equivalent litres (or cubic feet). Dividing by average absolute pressure at depth converts the actual depth breathing into a surface-equivalent rate. Dividing by time gives you litres per minute.
Worked example
A 12-litre tank goes from 200 bar to 80 bar over 45 minutes at an average depth of 18 metres (2.8 ATA).
- Pressure used: 200 − 80 = 120 bar
- Total gas at surface: 120 × 12 = 1440 surface-litres
- Average depth pressure: 1 + (18 ÷ 10) = 2.8 ATA
- SAC: 1440 ÷ 2.8 ÷ 45 = 11.4 L/min
That is excellent — top recreational divers run 10 to 14 L/min. Most certified divers start out at 18 to 25 L/min and improve with experience.
Typical SAC rate ranges
| Diver type | SAC rate (L/min) |
|---|---|
| Highly efficient (instructor / tech) | 8 to 12 |
| Experienced recreational | 12 to 16 |
| Average certified diver | 16 to 22 |
| New diver | 22 to 30+ |
| Stressed/cold/working hard | 30 to 60 |
Why SAC rises in practice
- Stress drops your SAC the fastest. New divers, current, low visibility, equipment problems — any of these can double the rate within a few minutes.
- Cold water. Below 18°C/65°F, shivering and increased ventilation push SAC up by 20-40%.
- Heavy work. Strong currents, finning hard, dragging gear all raise breathing rate.
- Poor trim. Vertical or head-down diving forces you to fin constantly. Horizontal trim drops SAC noticeably.
- Smoking and poor cardiovascular fitness correlate with higher SAC.
Using SAC to plan tank duration
Once you know your SAC, you can predict how long a tank lasts at any depth:
bottom_time = (usable_gas × tank_volume) ÷ (SAC × depth_pressure_ata)
A 12 L tank from 200 bar down to a 50 bar rock-bottom reserve is 150 bar usable, or 1800 surface-litres. At 18 m (2.8 ATA) and 15 L/min SAC, that gives 1800 ÷ (15 × 2.8) = 43 minutes. At 30 m (4 ATA) the same tank gives only 30 minutes. SAC stays constant; depth eats the gas.
Logbook habit
The single most useful long-term habit: write SAC in your log every dive. Trend it over 50 dives and the improvement is obvious — by the time most divers hit 100 dives, their SAC has dropped 30 to 50%. That is more bottom time, more comfort, and longer dives for free.