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Scuba Air Consumption (SAC) Calculator

Find your surface air consumption from a logged dive and use it to plan the next one. Enter your tank, the pressure you used, average depth and dive time to get SAC and RMV, then see the gas a planned dive needs, the pressure you should surface with and when to turn around.

Last updated:

Units
Your logged dive
cu ft

Aluminum 80 = 77.4 cu ft, HP100 = 100 cu ft

psi
L

Common sizes: 10, 12, 15 L (twin 12s = 24 L)

psi
psi
bar
bar
ft

Use the average depth from your dive computer log

m

Use the average depth from your dive computer log

min
Plan your next dive (same tank)
ft
m
min
psi
psi
bar
bar
Enter your values to see the result.
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How to use the Scuba Air Consumption (SAC) Calculator

  1. Choose units (imperial or metric) and salt or fresh water.
  2. Enter a logged dive: tank size, start and end pressure, average depth and dive time.
  3. Read your RMV and SAC rate, the gas you breathe per minute at the surface.
  4. Enter the planned dive: depth, bottom time, start pressure and reserve, and pick a planning factor.
  5. Check the plan: gas required, surfacing pressure, turn pressure and the bottom time table.

Scuba air consumption formulas

MeasureImperialMetric
Absolute pressure (ATA)depth ft ÷ 33 + 1 (fresh: ÷ 34)depth m ÷ 10 + 1 (fresh: ÷ 10.3)
Tank factorrated cu ft ÷ rated psitank water volume in liters (L per bar)
SAC ratepsi used ÷ (minutes × ATA)bar used ÷ (minutes × ATA)
RMVSAC × tank factor (cu ft/min)SAC × liters (L/min)
Gas neededRMV × ATA × minutesRMV × ATA × minutes
Pressure neededgas needed ÷ tank factorgas needed ÷ liters

Common tank sizes

TankCapacityWorking pressureTank factor
Aluminum 80 (AL80)77.4 cu ft3,000 psi0.0258 cu ft/psi
Aluminum 6363 cu ft3,000 psi0.0210 cu ft/psi
Steel HP100100 cu ft3,442 psi0.0291 cu ft/psi
Steel LP8585 cu ft2,640 psi0.0322 cu ft/psi
12 L steel≈ 2,400 L at 200 bar200–232 bar12 L per bar
15 L steel≈ 3,000 L at 200 bar200–232 bar15 L per bar

Check the markings and the manufacturer's data for your own cylinder; capacities above are typical published values. These formulas treat air as an ideal gas, which is close enough for recreational gas planning.

Tips for better gas planning

  • Measure over several dives. Use your computer's average depth and the total dive time, and average a few calm dives to get a dependable RMV.
  • Plan with a margin. Cold water, current, heavy gear or a new site raise your breathing rate. The planning factor lets you add 25% to 100%.
  • Plan for your buddy too. Your reserve should be enough to get two divers to the surface from the deepest point if one has a gas problem.
  • Agree the numbers before the dive. Share turn pressure and reserve with your buddy and use the more conservative diver's plan.

Frequently asked questions

What is SAC rate in scuba diving?

Surface air consumption (SAC) is how fast you use gas at the surface, at 1 atmosphere. Because you breathe denser gas at depth, it is found by dividing the gas used by the dive time and the average absolute pressure. It is usually given as pressure per minute (psi or bar per minute) for a specific tank.

What is the difference between SAC and RMV?

RMV (respiratory minute volume) is the volume of gas you breathe per minute at the surface, in cubic feet or liters per minute, and it is the same whatever tank you use. SAC in psi or bar per minute depends on the tank size. RMV = SAC × tank factor (cu ft per psi) or × tank volume in liters.

How do I calculate my SAC rate?

SAC = pressure used ÷ (dive time × ATA), where ATA = depth ÷ 33 ft + 1 in salt water (depth ÷ 10 m + 1 metric). Example: 2,100 psi used in 40 minutes at an average 45 ft (2.36 ATA) gives 2,100 ÷ (40 × 2.36) = 22.2 psi/min. With an aluminum 80 (77.4 cu ft at 3,000 psi) that is an RMV of about 0.57 cu ft/min.

What is a normal RMV for divers?

Many relaxed recreational divers breathe roughly 0.5 to 0.75 cu ft (about 14 to 21 liters) per minute at the surface, while experienced, calm divers can be lower. Work, cold, current and stress can easily double it, which is why plans use a safety factor.

What is turn pressure?

Turn pressure is the gauge reading at which you end the outward part of a dive and head back. On an out-and-back dive a common rule is to turn at (start pressure + reserve) ÷ 2. The rule of thirds, used for more demanding dives, turns after using one third of the gas and keeps the last third as reserve.

How do I work out how long my tank will last at depth?

Multiply your RMV by the absolute pressure at that depth to get gas used per minute, then divide the usable gas (start pressure minus reserve, converted to volume) by it. The table in this calculator does that for common depths and subtracts gas for the ascent and safety stop.

Is this calculator a substitute for a dive computer?

No. It is a gas-planning aid for trained divers. It does not track no-decompression limits, nitrogen loading or ascent rates. Always dive within your training, follow your computer and agree your gas plan with your buddy.

Are all the tools on FreeToolCabin tested for quality and accuracy?

Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.

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