How to Calculate SAC and RMV

SAC stands for Surface Air Consumption and RMV stands for Respiratory Minute Volume. Both numbers help you understand how much breathing gas you use, but they are not exactly the same.

Your SAC rate shows how much pressure you use per minute at the surface, usually in bar per minute. SAC is cylinder-specific, because the same pressure drop means a different amount of gas in a 10 liter, 12 liter or 15 liter cylinder.

Your RMV shows your actual breathing volume at the surface, usually in liters per minute. RMV is more useful for gas planning because it is not tied to one specific cylinder size.

In the examples below we use the metric system: meters, bar and liters per minute.

Knowing your SAC and RMV is useful for recreational divers and technical divers, because it gives you a much better starting point for gas planning. It does not replace training, experience, reserves or good judgment, but it helps you stop guessing.

Total Time Needed :

Minutes

Required Tools:

– A calculator

Information Needed

– Start pressure

– End pressure

– Dive time or measurement time

– Average depth or test depth

– Cylinder size in liters

Steps: how to calculate SAC and RMV

1. Check the Gas You Used

Subtract your end pressure from your start pressure.
Example:
200 bar – 150 bar = 50 bar used

2. Note the Depth

For a complete dive, use the average depth from your dive computer.
For a short test at a stable depth, use the depth where you stayed during that test.
For conservative gas planning, you can calculate using the planned working depth.

3. Note the Time

Use the total dive time for a full-dive calculation, or the exact test time if you measured your breathing rate during a specific part of the dive.
Example:
10 minutes

4. Calculate Bar Used Per Minute

Divide the gas used by the time.
Example:
50 bar / 10 minutes = 5 bar per minute

5. Convert Depth to ATA

ATA stands for Atmospheres Absolute. It is the total surrounding pressure at a certain depth.
At the surface, the pressure is 1 ATA. Every 10 meters of seawater adds about 1 ATA.

Use this formula:
ATA = (depth / 10) + 1
Example at 20 meters:
(20 / 10) + 1 = 3 ATA

6. Calculate SAC

Use this formula:
SAC = bar per minute / ATA
Example:
5 bar per minute / 3 ATA = 1.67 bar/min
Your SAC rate is 1.67 bar per minute.

7. Calculate RMV

Use this formula:
RMV = SAC x cylinder size in liters
Example with a 12 liter cylinder:
1.67 x 12 = 20 liters per minute
Your RMV is 20 l/min.

Example: How Long Does a 12 Liter Cylinder Last?

Question:
How long does a 12 liter cylinder last with 120 bar at 30 meters if your RMV is 18 l/min?

First, calculate the pressure at 30 meters:
(30 / 10) + 1 = 4 ATA

Then calculate gas use at that depth:
18 l/min x 4 ATA = 72 l/min

Now calculate the available gas:
12 liters x 120 bar = 1440 liters

Then divide total gas by gas use per minute:
1440 / 72 = 20 minutes

The answer is 20 minutes.

Important: this is a simple calculation example. It does not include reserve gas, ascent gas, buddy gas, current, stress, workload, decompression obligations or a proper turn pressure. Never plan a real dive by using every liter in the cylinder.

Is one SAC or RMV calculation enough?

No. It is better to calculate your SAC and RMV over multiple dives and in different conditions. Current, cold water, stress, workload, visibility, equipment setup and experience level can all change your breathing rate.

Is there a best SAC or RMV rate?

No. There is no perfect number for every diver. Some divers breathe less, some breathe more, and your own rate can change from dive to dive. The important thing is to calculate your own realistic average and use it properly.

What is the difference between SAC and RMV?

SAC is usually shown as pressure used per minute, for example bar/min. RMV is shown as volume used per minute, for example liters/min. SAC depends on cylinder size, while RMV is easier to use across different cylinder sizes.

Can I use SAC and RMV for dive planning?

Yes, but only as part of proper dive planning. SAC and RMV help estimate gas use, but you also need to include reserves, ascent gas, buddy procedures, depth, conditions and your training level.

Why does gas consumption increase with depth?

Because the deeper you go, the greater the surrounding pressure becomes. At 30 meters you breathe gas at 4 ATA, so you use roughly four times more gas than you would at the surface.

Use your SAC and RMV as smarter starting points for better gas planning, safer dives and more relaxed bottom time.

Safe and awesome diving,
Team Aquarius

SAC and RMV