How to Calculate Your SAC Rate and RMV

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:

1. Gas used: starting pressure minus ending pressure, in psi.
2. Dive time in minutes.
3. Average depth, from your computer.
4. 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.

This follows the worksheet's pressure-to-volume calculation. UTD materials use SAC and SCR for surface-normalized consumption; check the units rather than relying on the acronym alone.

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.

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 is a volume illustration only: it excludes reserve and travel gas and does not authorize that bottom time under the applicable Min Deco limit.

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. Using the simplified emergency profile in our calculator gives 800 psi rock bottom for this example, leaving 2,200 psi, about 57 cubic feet, above reserve. 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 you spend all your usable gas. This simplified estimate excludes gas already spent on descent and assumes steady depth and consumption. Use actual pressure and the team's turn plan; do not treat half this calculated time as a guaranteed shore-dive turn time.

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. Use the stressed emergency rate for the reserve example, increasing it if your team needs more. A low relaxed measurement does not justify reducing the emergency reserve.

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.