About the Scuba Air Consumption (SAC) Calculator
This scuba air consumption calculator works out your surface air consumption (SAC) rate and respiratory minute volume (RMV) from a dive you have already logged. Enter your start and end tank pressure, the cylinder size, the average depth and the bottom time, and it converts what you breathed at depth back to the equivalent rate at the surface.
Use the result to plan the next dive: enter a planned average depth and the reserve pressure you want to surface with, and it estimates how many minutes of gas the same tank gives you. It supports imperial units (psi, cubic feet, feet of seawater) and metric units (bar, litres, metres).
SAC changes with exertion, cold, stress and experience, so log several dives and use a conservative figure. This is a planning aid for recreational divers, not a replacement for your training, dive computer or the rule of thirds on overhead and technical dives.
With the default inputs, the sac rate (psi/min or bar/min at the surface) is 30.14. Change any value above to recalculate instantly.
How to use the scuba air consumption (sac) calculator
- 1Choose imperial or metric units.
- 2Enter start and end tank pressure from your logbook.
- 3Enter the tank size (rated cu ft and psi, or litres).
- 4Enter the average depth and the dive time.
- 5Add a planned depth and reserve to estimate gas time for the next dive.
Formula and method
Water pressure increases by about one atmosphere every 33 feet (10 metres) of seawater, and by Boyle’s law each breath at depth uses gas in proportion to the absolute pressure. Dividing the pressure you used per minute by the average absolute pressure (ATA) gives your surface air consumption rate in psi or bar per minute — but that figure only applies to the same tank.
RMV converts SAC to a volume that works with any cylinder: multiply by the tank’s rated volume divided by its rated pressure (imperial), or by the tank’s water volume in litres (metric, where 1 bar in a 12 L tank is 12 L of surface gas). Planned time is usable gas (start minus reserve) divided by RMV × ATA at the planned depth.
- ATA
- Atmospheres absolute at the average depth
- SAC
- Surface air consumption, psi/min or bar/min
- RMV
- Respiratory minute volume, cu ft/min or L/min
- P_start, P_end
- Tank pressure at the start and end of the dive
Worked examples
AL80, 3,000 → 1,000 psi in 30 min at 40 ft
40 ft is 40 ÷ 33 + 1 = 2.21 ata. 2,000 psi over 30 minutes is 66.7 psi/min at depth, or 30.1 psi/min at the surface. With 77.4 cu ft per 3,000 psi that is an RMV of 0.78 cu ft/min; 64.5 cu ft of usable gas at 60 ft (2.82 ata) lasts about 29 minutes.
Metric: 12 L tank, 200 → 70 bar in 45 min at 15 m
15 m is 2.5 bar absolute. 130 bar × 12 L = 1,560 L of gas; 130 bar in 45 minutes is 2.89 bar/min at depth, or 1.16 bar/min at the surface, an RMV of 13.9 L/min. 150 bar × 12 L = 1,800 L usable at 3 bar (20 m) gives about 43 minutes.
Frequently asked questions
What is a good SAC rate for scuba diving?+
Experienced, relaxed divers often have an RMV around 0.4–0.6 cu ft/min (12–17 L/min). New divers and anyone working hard or cold commonly use 0.7–1.0 cu ft/min (20–28 L/min) or more.
What is the difference between SAC and RMV?+
SAC is usually expressed as pressure per minute (psi/min or bar/min) and only applies to the tank you measured it on. RMV is a volume per minute (cu ft/min or L/min) and works with any cylinder size.
How do I calculate how long my air will last?+
Multiply your RMV by the absolute pressure at the planned depth to get gas used per minute, then divide the usable gas in the tank (start pressure minus reserve, converted to volume) by that figure.
Why does air run out faster at depth?+
Under Boyle’s law, gas is compressed by the surrounding pressure, so at 33 ft (2 ata) every breath uses twice as many molecules as at the surface, and at 99 ft (4 ata) four times as many.
Should I use freshwater or saltwater depth?+
The calculator uses seawater (33 ft or 10 m per atmosphere). Freshwater is slightly less dense — about 34 ft per atmosphere — so the difference is small for recreational planning.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.