How to Calculate SAC Rate: A Step-by-Step Guide

Last Updated on January 21, 2025 by Candice Landau

I used to struggle to calculate my SAC Rate because every agency I took a scuba course through—PADI, GUE, NAUI, TDI—explained it differently, or taught it poorly.

It wasn’t until I took my instructor course and learned to teach the SAC rate calculation to others, that I figured out how to make it simple and hopefully comprehensible.

If you think I’m wrong, I’d love to know. Leave me a comment and I’ll do my best to clarify any confusion.

>> Related Reading: Scuba Agnostic: AKA, Why I Train Broadly

In order to calculate your SAC rate, you need to know how to calculate a few other things. This is why it’s easy to get confused. It’s a multi-step process. You also need to know:

1. How to calculate your Depth Consumption Rate (DCR).
2. How to convert depth (in feet or meters) to Atmospheres Absolute (ATA).
3. And, how to convert your SAC rate in psi/min or bar/min to cubic feet or meters per minute (what is actually called RMV or Respiratory Minute Volume).

Before I dig into the math, let’s go over some simple terminology. Don’t be alarmed, you’ll pick it up in no time.

>> Related Reading: No Time? Check Out the SAC Rate Calculator

What is SAC Rate?

Surface Air Consumption (SAC) rate is a measure of how much breathing gas a diver uses per minute at the surface.

It is typically expressed in either psi per minute (imperial) or bar per minute (metric).

By knowing this rate, divers can more accurately plan how much gas they will use at a given depth and over a certain period of time.

Why do we bother learning to calculate SAC Rate?

• Gas planning: If you know your SAC rate you can decide if you’ll have enough gas for the planned bottom time and for any required stops.
• Stress and effort indicator: A higher SAC rate can signal that a diver is overexerting or stressed. For example, GUE considers* a good SAC rate to be anything below 0.75 ft3/min and anything above 1.0 ft3/min to be poor. If you’re inbetween that, your SAC rate is average.
• Consistency and experience: By tracking your SAC rate over time you can establish a personal baseline and thus, more effectively manage your gas.

*(From “1-Surface-Consumption-Imperial.pdf” used in GUE Fundamentals course)

A step-by-step guide to calculating your SAC Rate

Right, we’re ready to get into it. I promise I’ll make it as simple as possible and break it up into the different steps it actually is. If I—a not particularly mathematically-inclined person—can understand it, you can too. The example below uses imperial measurements. If you desperately need it in metric, let me know.

For the purposes of this example, you’re going to calculate the SAC rate for a diver who uses 700 psi in 10 minutes at a depth of 64 feet of freshwater. You should know that they are wearing an AL 80 scuba tank. More on that in a bit.

Before we dive into the calculations, to determine your SAC rate, here’s the information you will need to ensure you have (and this is also why you should keep a log book or record your dive data somewhere):

• Tank starting pressure (We don’t need this for our example as we’ve already been told the person consumed 700 psi. If we had not been told, however, we would need this in order to determine that number)
• Tank ending pressure (same as above)
• How long it took you to consume that pressure (starting pressure minus ending pressure) (700 psi)
• Average depth (64 feet in freshwater)
• Tank type and capacity (AL80, 80 ft³ at 3000 psi)

For an accurate SAC rate, in an ideal world, you’d calculate your SAC rate for a set depth, rather than your average depth. Though you can do it for your average depth, it will be more accurate if you take the time to determine your SAC rate for a set depth.

For example, if you want to determine your SAC rate at 64 feet, you’d swim at that same depth profile for 10 minutes (not going much above or below 64 feet). You’d take note of your starting pressure at 64 feet when you begin your 10 minute swim and then of your ending pressure at 64 feet when you hit the 10 minute mark.

Step 1: Determine the Depth Consumption Rate

Let’s recall the example prompt: Calculate the SAC rate for a diver who uses 700 psi in 10 minutes at a depth of 64 feet of freshwater. You should know that they are wearing an AL 80 scuba tank.

DCR =

DCR = 70 psi/min

So, the diver uses 70 psi/minute in 10 minutes.

Step 2: Convert the depth to Atmospheres Absolute (ATA)

The exact conversion depends on whether the diver is in freshwater or saltwater. Our example tells us we’re talking about freshwater.

If freshwater use 34 feet (1 ATA)

If saltwater use 33 feet (1 ATA)

ATA =

ATA =

ATA ≈ 2.88 ATA

And so, in 64 feet of freshwater, our diver’s ATA is 2.88. Now we have what we need to begin calculating SAC Rate.

Step 3: Calculate the SAC Rate

The equation for SAC Rate is simple. We take the answer we got in step 1 and divide it by the answer we got in step 2. Mathematically, it looks like this:

SAC =

SAC =

SAC Rate ≈ 24.3 psi/min

This means our diver’s SAC Rate is 24.3 psi per minute. This is how much gas they consume per minute. Now, most of the time you’ll see SAC Rate expressed in cubic feet per minute. Technically we refer to this as the RMV (respiratory minute volume). That’s the number most people are referring to when they talk about SAC Rate. Let’s talk about that in the next step.

Step 4: Convert to Cubic Feet per Minute

To convert from psi to cubic feet, we first need to know the tank factor of an AL 80 scuba cylinder.

This will differ depending on the tank you’re diving. To start, you’ll first need to know what your tank’s rated capacity is. The tanks rated capacity should be stamped into the tank near the top. However, here are some common sizes and capacities that you can keep as a cheat sheet of sorts:

• AL 80 = 3,000 psi, 80 ft³
• AL 63 = 3,000 psi, 63 ft³
• Steel 100 = 3,442 psi, 100 ft³
• Steel 120 = 3,442 psi, 120 ft³

Tank Factor =

Tank Factor = 0.0267 ft³/psi

Now that we have our tank factor for an AL 80, let’s find our SAC Rate in cubic feet per minute.

SAC Rate(cfm) = 24.3 psi/min x 0.0267 ft³/psi

SAC Rate(cfm) = 0.65 ft³/min

And so, our diver’s SAC Rate is 0.65 ft³/min, which according to GUE is a good SAC rate.

If you’re diving doubles

If you’re diving doubles, or a twinset, your SAC rate calculation doesn’t change. It’s still based on your personal air consumption at the surface and doesn’t depend on the size or type of tank but rather on your breathing rate and activity level.

What DOES change is the way you calculate the gas available to you and your depth consumption rate (DCR) since you now have two cylinders sharing the same regulator setup.

All you have to do is double your tank factor. So a set of 80 ft³ cylinders goes from 0.0267 ft³/psi to 0.053 ft³/psi. So, while your SAC Rate stays the same, your Depth Consumption Rate (DCR) now uses the combined tank factor (example: 0.053 ft³/psi for double AL 80s) because you’re consuming gas from two tanks simultaneously. This allows for more gas at the same PSI drop compared to a single tank.

You could write the equation like this if you wanted to:

Doubles Tank Factor = 2 × (Single Tank Factor)

And that’s it, simple!

Using SAC Rate in dive planning

Once you know your SAC Rate, you can apply it to different dive profiles and determine your total gas requirements for a specific dive. By multiplying your SAC Rate by the time you intend to spend at depth (adjusted for ATA) and adding a safety margin, you can ensure you are carrying sufficient gas. This is particularly important if you’re participating in technical diving, or in scientific diving, which often requires working underwater.

If you wanted to know how to do just this, all you’d have to do is flip the equation. So, it would become:

DCR = SAC Rate x

So, for example, if your SAC Rate is 20 psi/min and you want to know how fast you’ll use 1,500 psi at 65 ft in seawater, you’d plug it in like so…

DCR = 20 psi/min x

DCR = 20 psi/min x

DCR ≈ 59.39 psi/minute

Next, we take 1,500 psi and we divide it by 59.39 psi/min. We get 25.26 min.

This means that we’d be able to spend approximately 25 minutes at 65 feet on 1,500 psi.

Of course, all of this assumes your SAC Rate stays relatively constant on that dive and that you don’t have stressors. Make sure to always reserve room for emergency gas. Also bear in mind that you need to be using the same size tank for this dive as the dive that gave you your initial SAC Rate of 20 psi/min.

Get to know your SAC Rate over multiple dives and using different tanks. It will improve your confidence underwater and help you figure out what changes you need to make, especially if you find it’s in the poor range.

How can you improve your SAC Rate?

Ultimately, to improve your SAC Rate, you need to exert yourself less underwater and to remove stressors. You need to do all of this while maintaining efficient and relaxed breathing. Here are some strategies you can employ:

Ensure you’re properly weighted and diving in trim:

• Being either underweighted or overweighted can lead to unnecessary buoyancy adjustments with your BCD or wing and increased finning, both of which raise your SAC rate.
• Dial in your weighting and trim to move smoothly and effortlessly through the water.

Streamline your dive gear:

• Extra gear or dangling accessories create drag and force you to work harder.
• Use gear configurations that keep hoses, regulators, and accessories close to your body to minimize resistance.

Focus on relaxed breathing techniques:

• Practice slow, steady inhales and exhales to avoid short, shallow breathing.
• Keep your breathing rhythmic and avoid breath-holding, which can lead to CO₂ buildup and stress.

Improve your physical fitness:

• Cardiovascular fitness and overall strength help you handle exertion more efficiently.
• Regular exercise topside (swimming, running, strength training) can boost your overall endurance and reduce your air consumption underwater.

Move slowly and efficiently:

• Rapid or jerky movements will boost your heart rate and SAC rate.
• Plan your dive so you can stay calm and move methodically, rather than rushing from point to point. Try different finning techniques and experiment with how this changes your SAC rate.

Practice buoyancy control:

• Good buoyancy and neutral trim let you glide more and fin less.
• This reduces energy expenditure and makes your breathing more stable.

Stay warm:

• Getting cold can lead to tense muscles, shivering, and faster breathing.
• Using the right exposure protection for the water temperature keeps you comfortable and helps reduce your SAC rate.

Manage your stress and anxiety:

• Anxiety or mental stress leads to elevated breathing rates.
• Dive within your training and comfort limits, and use relaxation techniques if you start to feel tense.

At the end of the day, log your dives, the water temperature, the gear you used and so on. This is the best way to get a feeling for what is impacting your SAC Rate. And remember, if you’re a newer diver, it will improve in time.

Do you have questions about other mathematical calculations as they relate to scuba diving? Leave me a comment and I’ll do my best to explain SIMPLY in a fresh article.