SAC rate and RMV: calculate your air consumption and plan your gas
In short
Your SAC rate (surface air consumption), or RMV, is how much air you breathe per minute at the surface. At depth, multiply it by the absolute pressure: 20 L/min (0.7 cu ft/min) becomes 60 L/min (2.1 cu ft/min) at 20 m (66 ft). Divide your usable air, above your reserve, by that figure to estimate your time.
Twenty minutes into your first deep dive, your gauge reads far lower than on yesterday’s shallow reef, though you breathed just as calmly. At depth, every breath takes more air from the tank, and this page shows how much.
What are SAC rate and RMV?
SAC rate and RMV both describe how fast you breathe, brought back to surface pressure so dives at different depths compare. The words vary:
- SAC (surface air consumption), as DAN uses it, is a pressure rate: bar or psi per minute. It only applies to one size of tank.
- RMV (respiratory minute volume) is a volume rate: liters or cubic feet per minute. It works with any tank.
- Many divers, and Octo’s lessons, use both names for the volume rate.
DAN converts one into the other by multiplying the SAC by the tank’s rated volume: 2 bar per minute on an 11 L tank is an RMV of 22 L/min.
Why does depth multiply air consumption?
Depth multiplies air consumption because of Boyle’s law. Your lungs take the same volume with each breath at any depth, but at depth that volume holds denser air; DAN notes that Boyle’s law is why you inhale more and more gas molecules per breath as you descend. So consumption at depth = surface rate × absolute pressure.
| Depth (seawater) | Absolute pressure | At 20 L/min | Minutes on 1,800 L | At 0.7 cu ft/min | Minutes on 66.7 cu ft |
|---|---|---|---|---|---|
| Surface | 1 bar (1 ata) | 20 L/min | 90 | 0.7 cu ft/min | 95 |
| 10 m (33 ft) | 2 bar (2 ata) | 40 L/min | 45 | 1.4 cu ft/min | 48 |
| 20 m (66 ft) | 3 bar (3 ata) | 60 L/min | 30 | 2.1 cu ft/min | 32 |
| 30 m (99 ft) | 4 bar (4 ata) | 80 L/min | 22.5 | 2.8 cu ft/min | 24 |
Usable air: a 12 L tank from 200 to 50 bar (1,800 L), or a tank holding 80 cu ft at 3,000 psi, down to 500 psi (66.7 cu ft). The example tanks differ, so the minutes don’t match exactly. Ascent and safety stop not included.
Drag the depth: Boyle’s law in action
Try a question from the app
gas use at depth
You breathe at the same pace at 10 m and at 30 m. How much faster does your gas run out at the deeper depth?
Show the answer
Answer B. What counts is absolute pressure: 2 bar at 10 m, 4 bar at 30 m, so twice as much gas per breath. Four times is the comparison with the surface; three times divides the depths instead of the pressures.
How do you calculate your SAC rate?
You calculate your SAC rate by measuring the air you used in a known time at a steady depth, then dividing by the absolute pressure there. DAN suggests repeating the test swimming faster and slower, for hard and easy dives.
Worked example
Measuring your own rate. You spend 20 minutes at a steady 10 m (33 ft).
- Metric, 12 L tank: you used 60 bar, so 3 bar/min at 10 m. Divide by 2 bar: SAC = 1.5 bar/min. Times 12 L: RMV = 18 L/min.
- Imperial, tank holding 80 cu ft at 3,000 psi: you used 1,200 psi, so 60 psi/min at 33 ft. Divide by 2 ata: SAC = 30 psi/min. Each psi is 80 ÷ 3,000 cu ft, so RMV = 30 × 80 ÷ 3,000 = 0.8 cu ft/min.
How long will your tank last at depth?
Your tank lasts as long as your usable air divided by your consumption at depth. Usable air is everything above your reserve: plan to reach the surface with at least 50 bar (500 psi), or the reserve set in the briefing if it’s higher.
Worked example
How long at 18 m (60 ft)? Your RMV is 20 L/min (0.7 cu ft/min).
- Metric: 18 m is 2.8 bar, so 20 × 2.8 = 56 L/min. Usable air in a 12 L tank from 200 to 50 bar: 150 × 12 = 1,800 L. Time: 1,800 ÷ 56 ≈ 32 minutes.
- Imperial: 60 ft is 60 ÷ 33 + 1 ≈ 2.82 ata, so 0.7 × 2.82 ≈ 1.97 cu ft/min. Usable air from 3,000 to 500 psi in a tank holding 80 cu ft: 2,500 ÷ 3,000 × 80 ≈ 66.7 cu ft. Time: 66.7 ÷ 1.97 ≈ 34 minutes.
That’s before the ascent and safety stop.
Worked example
Checking your gauge during the dive. You started at 200 bar (3,000 psi). After 15 minutes at a steady depth, you read 140 bar (2,100 psi).
- Metric: (200 − 140) ÷ 15 = 4 bar/min. To the 50 bar reserve: (140 − 50) ÷ 4 = 22.5 minutes at the same depth and pace.
- Imperial: (3,000 − 2,100) ÷ 15 = 60 psi/min. To the 500 psi reserve: (2,100 − 500) ÷ 60 ≈ 27 minutes.
The estimate holds only at the same depth and pace; going deeper or working harder shortens it.
Try a question from the app
deeper than planned
You planned a dive at 12 m (40 ft) but spent part of it at 18 m (60 ft). What about your air?
Show the answer
Answer D. Deeper means denser air and faster consumption. Your plan’s timing no longer holds: check your gauge and adjust the dive.
What raises your air consumption?
Anything that makes you breathe harder raises your air consumption: depth, effort (DAN notes that swimming into a current pushes your rate above your calculations), cold, stress, and poor buoyancy that keeps you finning. New divers use more air; it usually drops with experience.
To use less: move and breathe slowly, get your weighting right, streamline your gear and stay warm. Never skip-breathe (holding each breath to save air): it saves little and lets carbon dioxide build up.
What are the common SAC rate mistakes?
- Using gauge pressure. At 18 m (60 ft), 1.8 instead of 2.8 gives 50 minutes instead of 32.
- Spending the reserve. The reserve covers problems at the end of the dive; never plan to use it.
- Using a SAC in bar/min with another tank. Convert to RMV first: metric vs imperial dive math shows why psi per minute only fits one cylinder.
- Planning on the better breather. The diver who uses air faster sets the length of the dive for the pair.
Which dive course covers air consumption?
In Octo, the Open Water Diver level teaches air consumption and depth, reserves and turn-around pressure. Advanced Open Water applies it to deep dives, and the Divemaster level adds SAC, RMV and gas planning. Every formula is in the dive theory formulas guide, and the Open Water practice questions include air at depth.
Frequently asked questions
What is a good SAC rate?
There’s no single good number: your rate depends on your size, fitness, experience, the water temperature and your effort. Octo’s lessons use 20 L/min (0.7 cu ft/min) as an example figure. Measure your own rate on easy and hard dives, and plan with the higher one.
How do I measure my SAC rate?
Note your tank pressure at the start and end of a period at a steady depth, for example 20 minutes. Divide the pressure used by the minutes, then by the absolute pressure at that depth: that’s your SAC. Multiply by your tank’s rated volume to get your RMV. Repeat on several dives, at different paces.
Why does my air run out faster at 30 m (99 ft) than at 10 m (33 ft)?
Because the absolute pressure is 4 bar (4 ata) at 30 m (99 ft) and 2 bar (2 ata) at 10 m (33 ft). Each breath holds twice as much air at the deeper depth, so the same tank lasts half as long. Compared with the surface, it lasts a quarter as long.
Should I use the rule of thirds?
DAN describes the rule of thirds for dives that return to a set ascent point, such as a boat’s anchor line: a third of the gas for the way out, a third for the way back, a third for delays or emergencies. Turn-pressure rules vary between agencies, dive centers and types of diving. At recreational level, agree on a turn-around pressure in the briefing and surface with at least 50 bar (500 psi).
