It seems many folks buy an air compressor without considering compressor sizing or how to go about it correctly.
Things to be considered include what they intend to use the air compressor for, or how big an air compressor they are going to need to provide adequate compressed air to run their air tools and compressed air using equipment?
For compressor sizing it is necessary to figure out how much air you’re going to need. That is… the air requirement for all the air tools, all the blow guns, all the air cylinders, all the air operated diaphragm pumps, all the everything that is in your plant, workshop or garage that uses (or has the potential to use) compressed air.
Consider how much of that equipment will be operated at the same time, and whether or not that compressed air demand is going to be continuous or intermittent, or change as your tool use changes, production using air equipment increases, and so on.
How Can Air Compressors be Sized?
Firstly, lets thing about how you can size your air compressor. 6 parameters instantly come to mind and these are:
- CFM (cubic feet per minute)
- Pressure (psi / bar)
- Tank (liters / gallons)
- Power (hp / kW)
- Physical dimensions (feet / metres / footprint / volume)
- Duty Cycle (specified as a percentage)
CFM (cubic feet per minute) is the normal unit of measure for the volume of discharge air or flow rate from a compressor over time. As you may have guessed, its units are based on; volume/time. Please visit our page for more information on this.
Pressure (measured in PSI – pounds per square inch) is very important within air compressor systems. We have working/operating pressure given as a guideline as to what the air compressors optimum performance will be at.
We also have upper and lower limits to depict when the air compressor should cut in or cut out, and must consider that if the difference between the cut-in pressure and the cut-out pressure is set too small, the compressor will quickly start and stop repeatedly.
The actual volume of the tank itself is of course a very common way of sizing air compressors, I will touch on this further in the worked example to follow. We have a separate page on sizing compressor air tanks here.
The power or horsepower (hp) is a measurement of the mechanical energy an air compressor uses to complete its task of compressing air. Typically air compressors have HP ratings anywhere between 1.5 and 6.5, but some larger stationary air compressors may have up to 15 HP.
The actual size of the whole air compressor system is probably the first thing that springs to mind when you think of the literal term “size” and apply it to an air compressor. These physical dimensions can be measured in a number of ways:
The compressor Duty Cycle is quite an important piece of information to have when you are sizing an air compressor for your home or business workshop.
A duty cycle of 50% for a particular brand and model of compressor would mean that this particular unit could run steadily for 10 minutes, and then it must have a 10 minute rest period before it kicks in to compress more air. More information on duty cycle can be found on our detailed page.
The Key Parameters when Specifying Air Compressor Size to Suit a Particular Tool
Within those 6 parameters just presented, CFM, pressure, tank size and duty cycle are the key parameters you should focus on when sizing your compressor.
Why? You may ask… Well the answer is simple:
CFM and pressure are a function of the power anyway, and power isn’t as important as pressure or flow in determining how well your compressor will work. Duty cycle as already mentioned is an important consideration, along with tank size (amount of air being stored), whilst physical size (actual size of the whole system) isn’t typically a major consideration unless talking about an air compressor that must fit in your car for emergencies – or you’re fitting out a compressor room in an industrial application.
I will provide you with a worked example in this article which will help you better understand why these 4 parameters are the most important.
How to Size an Air Compressor For Air Tools
Air tools have a CFM rating typically specified at 90 PSI – however that doesn’t mean the WORKING pressure of the tool is 90 PSI – you might actually find that the WORKING pressure of the tool is significantly less (less than half) than 90 PSI. This is an important consideration to look at when buying or sizing an air compressor for your desired application.
Why Duty Cycle Matters When Sizing an Air Compressor
Duty cycle is specified typically as a percentage – see our article all about duty cycle.
Assuming the duty cycle is specified at 50% that would mean that the compressor should run for a maximum of 10 minutes, then have 10 minutes of down time (see your own user manual for your duty cycle and time increment).
When figuring out what size compressor you need, a key consideration is making sure that the compressor stays within it’s duty cycle limits. It’s ENTIRELY possible that you could purchase a compressor of sufficient CFM for the tools you’re powering, but exceed the duty cycle of the compressor.
Thus you THINK you’ve sized the compressor correctly – but actually, you’ve chosen an underrated compressor for your application.
A Worked Example of Sizing an Air Compressor
This initial worked example I am providing you with is a highly simplified way of calculating whether you’re exceeding your air compressors duty cycle. Let’s say your:
- Air compressor has a 50% duty cycle
- Air tool has a demand of 5 CFM
- Air compressor delivery capability is 8 CFM
Under CONTINUOUS DRAW of 5 CFM, your compressor would have to work 5/8 of the time or expressed as a percentage 62.5% of the time…
So straight away, you’re exceeding the stated duty cycle of 50% for your air compressor.
This is massively simplified as I said because it takes no consideration of the air compressor tank size or storage pressure! But it’s a gives you a good underestimate.
A Worked Example – Increasing Air Compressor Tank Size to Stay within Duty Cycle
So now let’s take that same worked example but apply some thought about tank storage pressure:
- Air compressor has 50% duty cycle
- Air tool demand is 5 CFM
- Compressor delivery capability is 8 CFM
- Air tank storage pressure is 180 PSI and outlet pressure is 90 PSI
First, let’s assume the air tank is 15 liters / 0.5 Cubic Feet @ 90 PSI
The “atmospheric” air in the tank is 90/15 * 0.5 = 3 Cubic Feet
(this is because atmospheric pressure is 15 PSI give or take (it’s 14.696 PSI) – and “90 PSI” above atmospheric, is 6 x 15)
But we’re storing at 180 PSI… So we need to double this number from 3 cubic feet to 6 cubic feet…
With the compressor at full pressure (180 PSI) there is “theoretically” 6 Cubic Feet of air at 90 PSI stored in the tank (this is an oversimplification! But, twice the pressure means there’s twice as many air molecules in the tank than there would be at 90 PSI, thus, twice as many cubic feet!)
Of course, as the tank pressure needs to be above 90 PSI to deliver 90 PSI at the outlet, the “volume of air available” is once again halved – so instead of 6 Cubic Feet of air, it’s 3 Cubic feet of air…
So, now assuming the air tank is full at 180 PSI and your tool is CONTINUOUSLY drawing down 5 CFM at 90 PSI.
The tank will deplete to 90 PSI in 3/5 minutes = 0.6 minutes… Yes that’s 36 seconds.
NOTE: The compressor would of course cut in before we reached 90 PSI to once again fill the tank and keep it above the 90 PSI working pressure at the outlet.
At 8 CFM at 90 PSI capability the air compressor would have to work for a certain length of time to refill the tank.
Let’s assume you’re not drawing air from the compressor tank whilst it’s refilling for a second…
You’re trying to replenish 3 CFM into the tank… BUT you’re also replenishing ABOVE 90 PSI and for every PSI above 90 PSI in the tank, the compressor is going to be less efficient at compressing air. For ease (and oversimplification!) let’s call it 75% efficiency – so your 8 CFM compressor is now actually a 6 CFM compressor.
So your now 6 CFM compressor would pump 3 CFM of air in 3/6 of a minute or 30 seconds…
However, you’re also drawing air away from the system at 5 CFM at 90 PSI… So, your tank is being emptied and filled at the same time.
With your lower efficiency at above 90 PSI, your compressor is now only 1 CFM greater in capacity than your draw down.
So to ACTUALLY refill the tank with 3 CFM – and the compressor replenishing at 1 CFM – the 3 CFM replenishment volume would take 3 minutes to refill before cutting off…
Then remember our earlier calculation on how long it would take to empty the tank back down to 90 PSI?
That’s how long the compressor would switch off for – that’s just 36 Seconds…
So with this drastic over simplification – the compressor would run for 3 minutes, shut down for 36 seconds and then return to running for 3 minutes – assuming the cut in pressure was at or around 90 PSI…
This is VERY imperfect maths – but it serves a purpose to think about duty cycles under CONTINUOUS USE!
The compressor pump in this example would be running for 180secs/36secs+180secs = 83% of the time!
But the duty cycle is only 50% so this setup is VERY underrated…
So how do we fix this? Surely an 8 CFM compressor is big enough?!
As you’ll see from the above calculations – the CFM of the compressor is JUST one part of the equation…
Changing the Storage Tank Size
Let’s now assume that your storage tank is actually 60 Liters (2 Cubic Feet)
NOW The “atmospheric” air in the tank is 90/15 *
0.5 2 = 3 12 Cubic Feet
(again this is because atmospheric pressure is 15 PSI give or take (it’s 14.696 PSI) – and “90 PSI” above atmospheric, is 6 x 15)
But we’re storing at 180 PSI remember – so we’ve actually got 24 cubic feet of air
Once again though, we need to deliver at above 90 PSI so we have 12 Cubic feet of air available before the air in the tank drops below 90 PSI.
NOW with 12 Cubic Feet of air available and a continuous draw down of 5 Cubic Feet per minute
It would take 12/5 minutes (2 minutes 24 seconds) to deplete the air in the tank to 90 PSI!
So now the time that the compressor WOULDN’T be running would be 2 minutes 24 seconds – then it would run for 3 minutes (as per our previous calculation).
So the percentage “on time” would be 180/180+144 = 55%.
So we’re A LOT closer to the duty cycle limitations stated (50%) and we haven’t changed ANYTHING but the size of the air tank on the compressor from 15 liters (0.5 cubic feet) to 60 liters (2 cubic feet).
Worked Example Summary
The thing about these calculations is that they’re based on CONTINUOUS USE of the air tool – it doesn’t sit down and rest at ANY point in the cycle.
The reality is that unless you’re using a type of tool which sees continuous use – maybe something like a wet spray paint gun – basing the CFM, pressure rating and tank size (all worked around the duty cycle!) isn’t appropriate – it’s a very worst case scenario.
If the ratings of the compressor were JUST about ok for continuous use and if you believed that you were only likely to be using the tool for 50% of the time you could:
- Half the CFM of the compressor and still just cover the application, or;
- Half the tank size of the compressor – however this would likely increase the cycling on and off of the compressor which might not be ideal from a duty cycle perspective either – and the longevity of the compressor pump. If the compressor is going to be on and off every 30 seconds, that’s probably a little extreme.
- Half the max storage pressure above the working pressure (i.e. in the above example, working pressure was 90 PSI, storage pressure was 180 PSI, you could take 180-90 = 90 PSI , then half that 90 PSI to get 45 PSI. Leaving you with a storage pressure requirement of 90 + 45 PSI = 135 PSI storage pressure.
You could also size your compressor and tank (keeping within the duty cycle limitations) based on the continuous use case KNOWING that you’ve got plenty of head room to use it on a less than continuous use case.
These numbers are drastic oversimplifications and won’t be deemed accurate in the real world – but they are near enough real world approximations to help logically size an air compressor in terms of CFM, storage pressure and tank size – as ALL 3 need to be considered in relation to the DUTY CYCLE limitations specified by the manufacturer.
To Help Air Compressor Sizing Here Are Simpler Blocks of Information and Further Considerations
When first venturing into the air compressor-sizing arena, folks tend to be more than a bit intimidated. There’s such a vast array of compressors, of types, of capacities, with a veritable feast of accessories; it’s easy to become confused!
And, though there are many manufacturers of compressors world-wide, all are focused on selling their own products, which may impart a certain bias to the information that’s available from them about their compressor-sizing, you think?
So aside to the worked example I’ve provided you with, here are some more simpler blocks of information and answers to questions that may help you.
Consider What’s the Compressed Air For?
Have you determined what you’ll be using compressed air for? In order to correctly size your compressor, you really must do some homework towards figuring out the volume of compressed air you will require for your specific tools or amount of tools.
A number of pages are already available on our site for specific uses, these are:
- What size compressor do I need for impact wrench – published here
- What size compressor for sanblasting – published here
- What size compressor for stucco spraying – published here
- What size compressor for die grinder – published here
- What size air compressor for painting – published here
- What size air compressor for nail gun – published here
Is Compressed Air Use Quantity a Continuous or Intermittent Demand?
If it is necessary in your plans to run all of those compressed air using appliances at the same time, and for long periods of time, “continuously” so to speak, then you are going to require a much bigger compressor than if they are used intermittently and sporadically.
If your compressor is for continuous air use then start making a list of everything in your plant, workshop or home that uses air, and leave a space beside each to note the expected CFM demand for each. Better yet, use a spreadsheet so that the air consumption numbers can be set up to give you a running total of the air you’ll need.
Air Tool Consumption
Know that compressed air consumption of tools varies depending on the size of the air tool, it’s speed, and the efficiency of that air tool. You need to get the specific compressed air demand for that air tool from the manual or the manufacturer of the tool for accurate assessment of the overall compressed air demands of your shop.
Grinders will have changes in consumption due to grinding wheel size, and rotary sanders will have their consumption change due to the pad size. Even the same tool from two manufacturers will have different air consumption rates.
You may find the chart below handy in approximating the CFM requirements of an assortment of standard air tools. This is a general guide only for that standard 90 PSI pressure. Your actual consumption of compressed air will vary, depending on your tool size and design.
The chart above is just a rough guide, and may not accurately reflect the air consumption of the air tool planned to be used.
The best source is to get the air consumption figures for a specific air tool from that tools manual. Failing to find that, contact the manufacturer! They should be able to give you the exact consumption. If contacting the manufacturer, have the model number and serial number of the air tool at hand because they will most likely ask for that.
Consider “factoring in” the highest air consumption figures you find for the tools you expect to use. You may oversize your air compressor capacity by doing so, but under sizing the compressor you purchase may bring greater long term costs to your plant or workshop as you grapple with air tools or air cylinders that do not run as expected due to a lack of compressed air supply.
The sizing calculations we have presented above are under the assumption that you are using a reasonably lengthy air compressor hose, but if you’re using a particularly long hose or hoses linked together with multiple connections you will have to consider pressure drop along the length of your hose and the connections.
Industrial plants have air compressors generating excess pressure and flow for the inevitable over-line air pressure drop. This helps to ensure that they have sufficient compressed air pressure and volume at their end application to keep their equipment running satisfactorily. The compressor you pick has to have this capability if it is an issue in your plant.
Again, some manufacturing plants will maintain a 20-30 PSI “safety margin” in extra compressed air pressure from the receiver, to help ensure that none of the plant equipment gets starved for air whether through pressure drop, leaks in the line, too much equipment being run at the same time, miscellaneous use of compressed air and so on.
This may not apply to most home users as most home users will be using their air compressor that is running in their garage and will be stood around 3FT from it. If not, please consider it as advised.
How Much Room Can The Compressor Take Up?
As mentioned, air compressors with large storage tanks can take up a lot of space. It is important to choose an air compressor that fits in your desired space, if space is small find a smaller compressor etc. A 5-gallon portable compressor will fit in almost any location whilst a 50-gallon stationary compressor will not and will only be suitable in big enough spaces. However, this is not necessarily down to the gallon size, but more about the fact the bigger compressor will have a more powerful motor.
Available Power Source
When selecting the compressor it is also very important to know what power source you will use for running it. Air compressors can either be powered by an electric motor or gas engine.
Is the air compressor you need capable of being plugged into a standard home outlet? Well, most are! However, some larger electric motor compressors may require a source of volts up to 240.
A gas engine running your compressor can provide more freedom as you can have compressed air almost anywhere. They will tend to be found in locations where electricity is not readily available like on construction sites. Gas engines have advantages in their flexibility of location however, they tend to be quite big so the ability to move them may become quite difficult.
Other Compressed Air Use
Air tools are only one of many applications in a plant that consume air. It’s necessary to give consideration to all of the ways that you will consume air in your plant and home applications.
Some folks don’t consider an air-driven blow gun as an air tool, yet these can sometimes use more compressed air than expected. So it is important to be aware of these considerations before sizing your compressor.
About Published Compressor Flow Specifications
Be careful when you are getting the air flow specifications on compressors, as some manufacturers size their compressor discharge rates as if the unit was pumping free air. As air pressure in the tank or mains air is compressed, the compressor pump has to work harder. A compressor might show a CFM rating of 8 for example, but reading the fine print will tell you that it generates 8 CFM at a much lower pressure rating than the pressure you may require.
Industrial systems (actuators / air tools) often require about 90 PSI to work to their capacity. Some will require higher pressure, and some lower of course, depending on the work each has to perform. This is because air compressors are typically rated at 90PSI as you can see on this Amazon listing below of a BILT HARD Air Compressor that is rated 4CFM at 90 PSI.
You will be using an air regulator at the point of compressed air use, to dial down the pressure and run the air equipment at the lowest possible effective pressure level. You may save substantial energy costs in doing so. For more on regulators needed for this size of air compressor please visit our regulator guide.
Existing questions about Compressor Sizing
We are looking to install a quiet indoor compressor into a salon / barber shop and the air will be piped out to each station