How to Calculate Your SAC Rate and RMV

To calculate your SAC rate, divide the gas you used by the dive time, then by your average depth in atmospheres. In psi, that's your SAC for that cylinder. Multiply by the cylinder's rated volume and divide by its rated pressure to get RMV in cubic feet per minute, which works with any cylinder.

By Jeremiah Gardner · Last reviewed October 27, 2026

Your surface consumption rate is the single most useful number in gas planning. It turns "I'll probably have enough" into arithmetic, and it shows you whether your diving is getting more efficient.

You don't need a calculator app. You need your logbook and four numbers.

SAC, RMV, SCR: what's the difference?

All three describe how much gas you breathe per minute, adjusted to surface pressure so dives at different depths can be compared.

SAC (surface air consumption) is often expressed in psi per minute. It's tied to one cylinder: 20 psi per minute means different amounts of gas in an aluminum 80 and an HP100.
RMV (respiratory minute volume) is expressed in cubic feet or liters per minute. It works with any cylinder.
SCR (surface consumption rate) is the term in UTD's planning material, used as a volume rate like RMV.

For planning, a volume rate is more useful. For reading your gauge underwater, psi per minute is more practical. You'll use both.

How do you calculate it from your logbook?

You need four numbers from one dive:

Gas used: starting pressure minus ending pressure, in psi.
Dive time in minutes.
Average depth, from your computer.
Your cylinder's rated volume and rated pressure.

Then:

Average depth in atmospheres (ATA) = average depth in feet ÷ 33 + 1 (salt water).
SAC in psi per minute = gas used ÷ dive time ÷ ATA.
RMV in cubic feet per minute = SAC × rated volume ÷ rated pressure.

Whatever the acronym, check the units. Psi per minute only applies to one cylinder.

Worked example

An aluminum 80 (77.4 cubic feet at 3,000 psi). You start at 3,000 psi and finish at 1,500 psi after a 40-minute dive with an average depth of 40 feet.

Step	Calculation	Result
Gas used	3,000 − 1,500	1,500 psi
Average depth in ATA	40 ÷ 33 + 1	2.21 ATA
SAC	1,500 ÷ 40 ÷ 2.21	17 psi per minute
RMV	17 × 77.4 ÷ 3,000	0.44 cubic feet per minute

That's a relaxed rate. Your next dive into current might be very different, which is why one dive isn't enough.

How do you use it at depth?

Multiply your surface rate by the pressure at depth. UTD calls this your depth consumption rate.

Example from UTD's planning material: at 100 feet (4 ATA) with a swimming rate of 0.75 cubic feet per minute, you'd breathe 0.75 × 4 = 3 cubic feet per minute.

To turn that into gauge pressure, use the cylinder's tank factor, the cubic feet it holds per 100 psi. For an aluminum 80, that's 77.4 ÷ 3,000 × 100, about 2.6; UTD's material rounds it to 2.5. Then:

psi per minute at depth = cubic feet per minute ÷ tank factor × 100
3 ÷ 2.5 × 100 = 120 psi per minute at 100 feet.

At that rate, a full aluminum 80 lasts about 26 minutes at 100 feet: 77.4 ÷ 3. That's every cubic foot, with no reserve, which is exactly why you plan with rock bottom.

To run your own numbers, use the SAC rate calculator. It measures your rate from a dive and shows how fast your gauge moves at any depth, with every step shown.

How much bottom time does your gas give you?

Using UTD's worksheet:

Bottom time = usable gas ÷ consumption rate ÷ depth in ATA

Example: an aluminum 80 at 3,000 psi on a 60-foot dive. Rock bottom is 800 psi, leaving 2,200 psi, about 57 cubic feet, of usable gas. At 0.75 cubic feet per minute and 2.8 ATA, that's 57 ÷ 0.75 ÷ 2.8, or about 27 minutes before you reach rock bottom, if the whole dive is at 60 feet. Real dives aren't that tidy, so your gauge and your turn pressure decide, not the estimate.

For rock bottom and turn pressures, see Gas Management for Recreational Divers and the Rock Bottom Calculator.

Measured rate or planning rate?

Both, for different jobs.

Your measured rate tells you how you're diving. Track it over time; if it drops, your weighting, trim and kicking are getting better. If it jumps on a dive, ask why.
Planning rates keep your reserves honest. UTD plans with 0.5 cubic feet per minute at rest, 0.75 while swimming and 1.0 under stress. Reserves use the stressed rate, however good your measured rate looks.

The logbook method

After each dive, record:

Cylinder (and its rated volume and pressure)
Start and end pressure
Dive time and average depth
Conditions: cold, current, workload, stress
Your calculated SAC and RMV

After ten dives, you'll know your normal range. After thirty, you'll see what changes it. If you want to lower it, see How to Improve Air Consumption.

Turn numbers into feedback

Our Scuba Coaching program uses exactly this kind of data, alongside video, to build a training plan around your diving.

Frequently asked questions

What's the difference between SAC and RMV?

Both describe how much gas you'd use per minute at the surface. SAC is often given in psi per minute, which only applies to one cylinder. RMV is in cubic feet or liters per minute, so it works with any cylinder. UTD calls the volume rate surface consumption rate, or SCR.

What SAC rate should I plan with?

For planning, UTD uses 0.5 cubic feet per minute at rest, 0.75 while swimming and 1.0 under stress. Use your measured rate to understand your diving, and plan reserves with the stressed rate.

How accurate is a logbook SAC calculation?

Accurate enough to compare dives and spot trends, especially if you use the average depth from your computer. Pressure gauges and temperature add some error, so look at several dives rather than one.

Does my computer calculate SAC for me?

Many air-integrated computers do. Check whether it reports psi per minute or volume per minute, and whether it uses your cylinder's correct rated volume.