CFM Explained: Understanding Air Compressor Output Capacity

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Air compressor with text overlay reading "CFM" in large bold black letters.

When it comes to abrasive blasting, airflow is king. If we had to use only one specification to recommend an air compressor for a blast cabinet, it would be airflow output capacity. This is probably the most important specification on a compressor’s spec sheet. The entire job of an air compressor is to provide a flow of compressed air. Compressor capacity tells us how well the compressor does that job. It is typically listed in some form of CFM, or cubic feet per minute. You’ll often see output capacity described as “CFM rating”, “Airflow Output”, “Airflow Delivery” or something similar.

Don’t confuse airflow output capacity with air tank storage capacity, which is typically listed in gallons.

What is CFM?

CFM is an acronym for Cubic Feet Per Minute. Think of it as a generic term. It describes the volume, or amount, of air an air compressor can put out in a one minute period of time. That’s easy right? Just pick the compressor with the highest rating. Maybe. Things are never that easy with air compressors. It depends on how the manufacturer is listing compressor capacity and what type of CFM we’re referring to. For instance, Displaced CFM isn’t the same as actual CFM.

CFM = Cubic Feet Per Minute
CFM is a measurement of how much air flows from an air compressor. Read on for more info on the different types of CFM and which is most important for blasting!

Describing an airflow output of 15 cubic feet per minute doesn’t tell us a lot. When does the compressor make that output? Is it just the maximum output possible? At what pressure or operating conditions does this output occur?

Air compressor output depends upon a variety of factors. Simply turning the pressure regulator up or down a few psi will change output. You’ll get more airflow volume at 40 PSI than you will running at 90 PSI. Ambient air temperature, elevation, air pressure, barometric pressure, humidity and a variety of other factors can all affect compressor performance.

When trying to evaluate performance ability, it is beneficial to know how a CFM rating was determined and under what conditions. We need to know more about what the compressor can do under specific conditions. It makes shopping for an air compressor somewhere like amazon.com much easier.

Fortunately, manufacturers have more specific air compressor capacity ratings to help with that. You’ll frequently see a letter in front of the abbreviation for cubic feet per minute. For instance: ACFM or SCFM. These other acronyms are different types of specifications related to compressed air volume. The letter in front tells us more about how the rating was obtained or what it is describing.

Let’s take a look at the different types of air flow specifications you may commonly encounter. I hope you’re ready for acronym overload!

  • DCFM: Displacement (or Displaced) Cubic Feet Per Minute
  • SCFM: Standard Cubic Feet Per Minute
  • ACFM: Actual Cubic Feet Per Minute
  • ICFM: Inlet Cubic Feet Per Minute
  • FAD: Free Air Delivery

That looks like a lot to keep track of. I’ve got good news. We really only care about one or two of these. That doesn’t mean the other ratings aren’t important. They are. Exploring each one gives you a better understanding of output capacity and CFM ratings, so let’s get started on doing just that.

DCFM – Displacement CFM

The D stands for “displacement”. This is the theoretical maximum output an air compressor should be capable of. Essentially the manufacturer calculates the amount of air a compressor can displace or draw into the compression chamber in one complete rotation cycle of the compressor’s crankshaft. DCFM ratings are based on the size of the compressor’s air cylinder, or cylinders, and the length of the stroke.

You will typically see a displacement rating listed as a number without a pressure or other associated conditions.

For example: 15 CFM or 15 DCFM.

When you see these specifications you need to keep looking and dig for more information. Why? They don’t tell us enough. This is all theory on paper. It is the maximum output an air compressor would be capable of in a perfect world with no losses or inefficiencies.

You’re pretty intelligent. You already see a major flaw with a compressor displacement rating. It is strictly a volume calculation based on the physical dimensions of compressor components. That’s fine but we don’t live, or blast, in a perfect world. Air compressors aren’t 100% efficient. There are losses due to heat, leaks, inefficiency and more. A displacement rating isn’t taking air pressure or environmental conditions into account. Like much of life, expectation doesn’t meet reality in the world of compressed gases. Actual delivered airflow numbers will always be smaller than the calculated displacement volume. A reduction of 1/3 is common.

DCFM is the theoretical maximum airflow output of a compressor. These ratings are derived from physical measurements and a calculation. They provide little to no information about actual airflow output in real world use.

To sum it up, DCFM is a theoretical calculation and pretty useless when shopping for air compressors. It is generic, idealized number telling us the maximum output a compressor would be capable of in a perfect world. Knowing the max possible airflow output helps a little, but it doesn’t help describe real world performance. We only know actual airflow output will be less. How much less? Who knows. It depends on the compressor and other factors. We need more information. This is where our other acronyms shine.

ACFM – Actual Cubic Feet Per Minute

The letter A stands for “actual” here. ACFM is Actual Cubic Feet per Minute. Now we’re getting somewhere! We aren’t just measuring some parts and doing some math. We’re taking a look at actual air compressor performance based in reality. This is a real-world measurement describing the actual volume of air a compressor delivers under specific operating conditions.

As you’d expect, these ratings are much more valuable than displacement ratings. As a measurement of actual airflow delivered, they take losses and environmental conditions into account. This specification provides valuable insight into compressor output in actual use scenarios. Knowing what a compressor does in the real world is much better than comparing fantasies on paper.

ACFM ratings are typically listed along with a description of the conditions the output readings were obtained under. For example:

15 ACFM @100 psi, 80 °F and 50% relative humidity.

These specifications provide much more relevant detail on real world airflow output. While ACFM ratings are an improvement over displacement calculations, there’s still a problem. There’s no common reference point.

Comparing compressors with these ratings is difficult if manufacturers are using different conditions. If one manufacturer is rating airflow output at 100 psi , 90 °F and 50% humidity how does another compressor rated at 120 psi, 70 °F and 0% humidity compare? That leaves a lot up to interpretation and a lot of guessing for the consumer. Fortunately, SCFM came along to help make things easier.

SCFM – Standard Cubic Feet Per Minute

The concept of Standard Cubic Feet per Minute (SCFM) was born to make comparison shopping for air compressors much easier. The letter “S” means standard here. This is an attempt by manufacturers and industry standards organizations to establish a common reference point for comparing air compressor output delivery.

Instead of attempting to compare multiple air compressors under a variety of differing conditions, SCFM provides a set of standardized conditions for comparison. If all compressor output capacity ratings are measured under the same, or very similar, conditions it makes things easier.

Here in the United States the standard conditions used are typically:

  • Temperature: 68°F (20°C)
  • Pressure: Sea level pressure (14.696 psi or 101.325 kPa)
  • Humidity: 0% relative humidity

Since the other factors are set, the major remaining variable is the pressure of the compressed air. As a result, you’ll usually see SCFM numbers listed at a specific pressure. The majority of air tools operate at a pressure of 90 psi. As a result, listing output at 90 psi is very common.

For example: 12 SCFM @ 90 psi.

Generally, a reciprocating air compressor’s airflow delivery will be higher at lower pressures. To give you a better understanding of compressor air flow delivery at different pressures, a manufacturer may provide multiple ratings at different pressures. For example air flow might be 11 SCFM @ 40 psi and 9 SCFM @ 90 psi.

SCFM = Standard Cubic Feet Per Minute
This is probably the best method we have to rate air compressor airflow output. This is the output rating to look for when shopping. This specification provides an idea what sort of airflow volume to expect at a certain air pressure under standardized conditions.

SCFM is probably the best we have but it still isn’t perfect. The concept is one set of standardized conditions across the board. There may be variation in practice however. There are regional differences, particularly between different countries. Some manufacturers may use slightly different conditions. It is important to refer to the manufacturer for more details on the specific conditions they use in their ratings. In the end minor these differences probably won’t matter all that much for the average end user consumer. The bottom line is standardized output ratings make comparing airflow delivery of different air compressors much easier since there is a common reference point.

FAD – Free Air Delivery

Free Air Delivery is a specification used to describe the actual volume of air delivered by an air compressor converted back to to the inlet conditions of the compressor. FAD ratings can have some ambiguity. They can be specified using local ambient operating conditions at the time of measurement or specified using standard conditions. This depends on the manufacturer and regional practices.

FAD and SCFM are sometimes used interchangeably, but they may not necessarily be the same. If FAD is measured under arbitrary local conditions it is similar to the concept of ACFM and might not be all that useful when comparing equipment. If FAD is provided under standardized conditions it becomes comparable to SCFM ratings.

It is imperative to consult the manufacturer for full details on the conditions used for the FAD rating if relying on the rating for comparing different compressors. Here in the U.S. we typically use SCFM to describe airflow delivery. FAD is more common in Europe and other areas of the world.

ICFM – Inlet Cubic Feet Per Minute

ICFM stands for Inlet Cubic Feet Per Minute. Instead of measuring airflow output, this is a rating of the volume of air moved into the compressor. This volume is typically measured on the inlet side and gives an idea of the volume of air moved under specific conditions just prior to entering the compression process.

ICFM is most commonly used with centrifugal compressors and not the typical reciprocating air compressor you’re probably familiar with. This is a measurement of air going into the compressor before the compression cycle occurs. As such, it typically doesn’t account for compressor inefficiencies, leakage or system losses. Knowing one compressor can suck in more air than another one might provide some insight but it doesn’t tell us much about the volume of air actually being delivered. These inlet ratings aren’t as useful as other ratings for the typical consumer hoping to compare air compressor performance.

How Much SCFM is Good for Blasting?

Now that we’ve got all the acronyms and abbreviations out of the way, you probably realize we’re most concerned with SCFM or FAD. Here in the U.S. the former is used most commonly. So how much SCFM is enough for a blast cabinet?

As a bare minimum starting point, for blasting alone, we recommend an air compressor with no less than 10 SCFM @ 90 psi. Keep in mind we say that is the bare minimum. You need at least double digit CFM output to even begin to do any productive sustained blasting. Getting by with less airflow output is possible but not fun, not productive and not recommended. Generally, more available air is better for blasting. We recommend purchasing a compressor with the largest airflow capacity your power source, budget and space allow. Get as far beyond 10 SCFM @ 90 psi as you can reasonably manage.

We know. That recommendation is simple and not exactly specific. For a more exact estimate on airflow requirements you’ll need to know your total air tool consumption and do a little basic math.

Calculating Air Tool Consumption and Airflow Requirements

This set air tool set from EXELAIR features 5 air tools, all with differing air tool consumption ratings. You’d need to size your air compressor to meet the needs of the tool with the highest air demand. If you plan on running multiple tools simultaneously, you’ll add up the total air tool consumption. (Air Tools shown above sold at HomeDepot.com)

Sizing an air compressor for a blast cabinet, or any air tool, means knowing the airflow requirements of the tool. Every air tool should have a listed air consumption specification. For example, a straight die grinder might have an air consumption rating of 6 SCFM @ 90 psi. That means you’ll need a compressor that can provide at least 6 SCFM @ 90 psi.

If more than one air tool will be in use simultaneously, your airflow requirements are more demanding. You need to account for all air tools that will be in use simultaneously. For instance, suppose you will routinely have a blast cabinet consuming 12 CFM @ 90 psi and a die grinder using 6 CFM @ 90 psi going at the same time. In this example you’d need a compressor that reliably provides at least 18 CFM @ 90 psi.

The Buffer/Fudge Factor

Once you’ve calculated your total air flow consumption you’ll have a starting point. Of course, matching your air flow consumption with your compressor output in an exact one to one ratio is a bad idea. Leakage and losses are common. You should choose an air compressor with slightly more airflow output than you need to provide a buffer. You need that “fudge factor” to give yourself some breathing room. This helps make using the tools practical. Having more airflow than you need avoids starving tools for air or overworking the compressor. How much more? How do we do calculate this?

To keep things simple, add up all of the air flow requirements for the tools that will be used simultaneously. Once you have that number, multiply it be at least 1.3 to 1.5 to account for losses and additional future demand.

For instance, imagine you have a blast gun that requires 9 cubic feet per minute of air at 90 pounds per square inch pressure. Your minimum compressor airflow output recommendation would be somewhere in the 11.7 to 13.5 SCFM range.

9 x 1.3 = 11.7
11.7 SCFM @ 90 psi = minimum

9 x 1.5 = 13. 5
13.5 SCFM @ 90 psi = recommended

This gives you a better idea of what sort of compressor to buy. You might get by with 10 SCFM at 90 psi. An output of 12 would be better. A compressor providing 14 or more SCFM at 90 psi would be better yet. However, an air compressor this powerful is often not practical for a normal homeowner garage or small shop due to power requirements and cost.

We’ve put together a separate post to help you determine how much CFM you need for blasting and more.

Horsepower, Airflow and Electrical Requirements

The amount of airflow a compressor can provide is determined primarily by the size of the motor and pump. More airflow requires larger pumps that must be driven by higher horsepower electric motors.

There’s a general rule of thumb when it comes to horsepower and compressor capacity. Generally, you should expect 3 to 4 CFM per actual horsepower. That makes 3.5 cubic feet per minute per horsepower a reasonable guess. We’re talking about actual running horsepower here and not peak ratings. We wrote an entire article dedicated to air compressor horsepower. We highly recommend reading it for more information.

More horsepower generally means better airflow out. The problem is it doesn’t take long to reach power source limitations with electric air compressors. The available electrical service is a key factor. To get 10 cubic feet per minute of airflow or better reliably from a single electric air compressor you’ll need more than 120V electrical. You’ll typically need 220-240 V power and at least a 3HP motor. In the United States, most hobbyists, DIYers and small businesses are going to have single phase power. This further limits how powerful of a motor you can run.

Furthermore, higher horsepower compressor motors draw more current and often need larger wire and circuit breakers for safe operation. Check with a qualified electrician and the air compressor manufacturer before making a purchase to ensure a compressor will meet your needs.

Conclusion

The capacity of an air compressor refer to how much airflow it can provide. It is one of the most important specifications to consider when choosing an air compressor for a blast cabinet or any other air tool. It is typically described using CFM or cubic feet per minute. Manufacturers may provide a calculated or measured output. Measurements of the actual volume of compressed air delivered at standard conditions are most valuable when evaluating equipment.

CFM (Displacement) is a theoretical calculation. It uses the physical size of the air compressor’s components to determine the maximum air flow delivery volume possible. It’s a paper calculation and provides a volume you won’t see in real life. Losses and inefficiencies are very real.

Air compressor capacity ratings listing actual air flow delivery measured under specific conditions are much more valuable when comparison shopping. Standard Cubic Feet per Minute is the most common, and probably most valuable, air compressor capacity rating we have here in the United States. SCFM accounts for environmental factors, inefficiencies and losses. It is a measurement of the actual volume of air a compressor is capable of delivering under specific standard operating conditions.

Abrasive blasting demands a high volume of air flow. Generally speaking, the more sustained airflow your compressor can provide, the better suited for blasting it is. We don’t recommend attempting to blast with anything less than 10 SCFM. For a better estimation of how much airflow output you need, add up the air consumption of all your tools that will be in use simultaneously. Next, multiply that number by 1.3 to 1.5. This calculation provides your airflow requirements with a little extra buffer built in to account for losses and future needs.

We hope this post has helped you have a better understanding of air compressor capacity ratings. Understanding the different compressor air flow ratings can help make purchasing a compressor for a blast cabinet much easier. In our opinion, based on many years of experience, proper CFM output of an air compressor is a critically important specification. Of course, compressor capacity is just one of many factors you need to consider when shopping for an air compressor. Our blog is packed with many other articles about air compressors and blasting. Our products and site are put together with one goal in mind: to help you blast big! Be sure to check our other articles, browse our products and feel free to contact us if you have any questions.

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