SAC Rate Explained: How to Calculate Your Gas Consumption

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What SAC rate means

SAC stands for surface air consumption. It's the rate at which you use gas, converted to what it would be at the surface. The conversion matters because gas at depth is compressed. At 10 meters (33 feet) of seawater, the absolute pressure is about twice the surface pressure, so every breath uses roughly twice as much gas from your tank as the same breath at the surface.

Converting to surface pressure puts all your dives on the same scale. A shallow reef dive and a deeper wall dive can be compared directly, and you can see whether your breathing is improving, or getting worse in certain conditions.

You'll see two related terms:

• SAC rate: usually expressed as tank pressure per minute (psi/min or bar/min). It depends on the tank you used.
• RMV (respiratory minute volume): expressed as gas volume per minute (cubic feet or liters per minute). It doesn't depend on tank size, so it's more useful for planning.

Many divers and dive computers use "SAC" loosely for both. Check which one your computer or app displays.

The SAC formula

Here's the standard formula. It's taught in many advanced and technical courses and used in most dive-planning apps.

Average absolute pressure comes from your average depth, not your maximum depth:

Most dive computers show average depth in the log, so you don't have to estimate it.

Worked example in psi (hypothetical numbers)

These numbers are made up for illustration. They aren't typical, recommended or safe figures for anyone.

A diver uses an aluminum 80 tank. The dive lasts 30 minutes at an average depth of 33 feet. The tank starts at 3,000 psi and ends at 1,500 psi, so 1,500 psi was used.

1. Pressure used per minute: 1,500 ÷ 30 = 50 psi per minute at depth.
2. Average ATA: (33 ÷ 33) + 1 = 2.
3. SAC: 50 ÷ 2 = 25 psi per minute at the surface.

That 25 psi/min only applies to that tank. To make it portable, convert it to RMV.

Converting SAC to RMV

To convert, you need the tank factor: the volume of gas per unit of pressure. It's the tank's rated volume divided by its rated pressure. Check your tank's markings or the manufacturer's spec sheet. A common aluminum 80 is listed at about 77.4 cubic feet at 3,000 psi.

Continuing the example: tank factor = 77.4 ÷ 3,000 ≈ 0.0258 cubic feet per psi. RMV = 25 × 0.0258 ≈ 0.65 cubic feet per minute.

Worked example in metric (hypothetical numbers)

In metric, the math is simpler because tank size is the water volume in liters, and gas volume is liters × bar.

A diver uses a 12-liter tank for a 30-minute dive at an average depth of 10 meters. The tank starts at 200 bar and ends at 100 bar, so 100 bar was used.

1. Gas used: 100 bar × 12 liters = 1,200 liters.
2. Gas used per minute at depth: 1,200 ÷ 30 = 40 liters per minute.
3. Average ATA: (10 ÷ 10) + 1 = 2.
4. RMV: 40 ÷ 2 = 20 liters per minute at the surface.

How air-integrated computers report SAC

Air-integrated computers read tank pressure from a transmitter, and they already know your depth and time. That's everything they need to calculate SAC or RMV. Most show it in the log after the dive, and some show it live. Common labels include SAC, RMV, gas consumption, or a breathing-rate graph.

To get accurate numbers:

• Enter the correct tank size in the computer or app. Without it, the computer can only report pressure per minute, or it will get RMV wrong.
• Pair the transmitter properly and watch for dropouts. See pairing AI transmitters.
• Read the per-segment view if your app has one. It shows where your breathing rate spiked, such as during a descent into current or a mask clear.

Many air-integrated computers also display gas time remaining, which estimates how long your gas will last at your current rate. It changes as your breathing changes, so treat it as a trend. Our air-integrated dive computers explained guide covers the feature in more depth, and best air-integrated computers lists models.

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Using SAC for planning

Once you know your RMV, you can estimate how much gas a planned dive needs. This is a planning check, not a substitute for the gas-management rules your agency and operator teach.

Planning example (hypothetical numbers)

A diver with a 20-liter-per-minute RMV plans 40 minutes at an average depth of 20 meters (3 ATA). Gas needed: 20 × 3 × 40 = 2,400 liters. In a 12-liter tank, that's 200 bar before any reserve, which means a single tank filled to 200 bar wouldn't cover the plan with any reserve. The dive plan needs to change: shorter, shallower, or a larger tank.

That's the value of the math. It shows before you get wet that the plan doesn't fit your gas.

Plan with your worst-case SAC

Your RMV on a relaxed, warm, shallow dive is not your RMV in cold, current or stress. For planning, many divers use a higher number than their average, such as their SAC from their hardest recent dives. Stress and exertion can raise your breathing rate a lot, and that's exactly when you need gas.

What affects your SAC rate

• Workload: swimming against current, finning hard or carrying gear.
• Temperature: cold raises breathing rate. See cold water dive gear.
• Stress and experience: new divers and anxious divers breathe faster. It usually drops with experience.
• Buoyancy and trim: poor weighting and trim make you work harder. See proper weighting.
• Body size and fitness: larger bodies and lower fitness generally use more gas. See dive fitness preparation.
• Equipment: leaky O-rings, free-flowing regs and poorly fitted masks waste gas. Keep your gear serviced. See regulator service explained.

How to improve your SAC rate

1. Dive more. Experience is the biggest factor for most divers.
2. Fix your weighting. Being overweighted makes you work harder and use more gas for buoyancy.
3. Streamline. Clip off dangling gear, and tidy hoses.
4. Slow down. Move slowly and use efficient fin kicks.
5. Stay warm. Better exposure protection reduces cold-driven breathing.
6. Breathe calmly. Slow, full breaths are more efficient than fast, shallow ones. Don't skip-breathe: holding your breath to save gas is unsafe and taught against in training.

Track your SAC over time

Log your SAC or RMV for every dive, along with conditions: water temperature, current, depth, exposure suit and how you felt. After ten or twenty dives, you'll see your baseline, your range, and which conditions push your breathing up. Many dive log apps graph this automatically. See how to log dives with your computer and app.

If you don't have an air-integrated computer, use a notebook or slate. Record the start and end pressure from your gauge, then use dive time and average depth from your computer.

Mistakes to avoid

• Using maximum depth instead of average depth. It makes your SAC look lower than it is.
• Comparing SAC in pressure units across different tanks. Use RMV instead.
• Planning with your best-ever SAC. Plan with a realistic or worst-case number.
• Entering the wrong tank size. Your computer's RMV will be wrong.
• Treating gas time remaining as a promise. It's an estimate based on your recent breathing.