How to Calculate the CFM of an Air Compressor

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Doing math can be frustrating except for the nerds. But when it comes to saving money, being a nerd can give you a little edge. Nevertheless, the calculations are so easy that you will need only your calculator app if you can’t do it in your mind.

And you need to do that because manufacturers can give you false air compressor output rates (CFM). Getting the wrong air compressor can be a costly mistake.

Key things to know before you get to the math

Now, most of you might have a good idea about the following measurements. You guys can skip it.

Noobs should read this part carefully.


It stands for pounds per square inch. This tells you the amount of force on a square inch of space. It’s the benchmark to determine the air force supplied by air compressors. 40-90 psi is the standard value for air compressors and most of the other air tools.


The full form of SCFM is Standard Cubic Feet per Minute, and CFM is only Cubic Feet per Minute. These units tell you the air output rate or the speed of air supplied. The main difference between the CFM and SCFM is that CFM is used for 90 PSI or less, and SCFM is used for 90+ PSI.

Horsepower (HP)

This unit is for measuring the power generation capacity of a motor. The greater the compressor horsepower, the more powerful the compressor.


This unit tells you the amount of air inside the tank. The compressor airflow depends on the volume. So, it is very important in calculating the CFM.

Calculation Process

After the calculation process, there is an example. For some people looking at the example first might make it easier to understand.

Anyway, the formula for calculating is-


V= Volume of air in the tank in cubic feet

P= Differential pressure

T= Tank refill time in minutes

Now you have to follow the following steps-

  1. Find the volume capacity of your compressor. It should be marked on the tank. If it is in liters, you have to convert it into gallons.
  2. To convert the value of the volume in gallons, multiply the value by 0.132. (This is the V in the equation)
  3. Now, you have to find the refill time of the tank volumes. Before emptying the tank to refill or the compressor discharge, note the PSI reading from the gauge. Then empty the tank completely. Open the stopwatch of your phone. Now refill the tank and note the time it takes to refill. (This is the T in the equation)
  4. Now find the difference in pressure. This is the difference between the PSI reading you took before emptying the tank (starting pressure) to the PSI reading you took after refilling the tank. You might also want to make sure that your pressure gauges are working properly.
  5. Lastly, convert the pressure to atmospheric pressure by multiplying the pressure difference by 0.968. (This is the P in the equation)

Now that you have found the three variables apply them in the formula to have the compressor CFM rate.

Here is an example to make it more straightforward.


  • Suppose you have an air compressor tank, and its air volume is 500 liters. Now divide the value with 3.785 to convert it into gallons, which should be 132.1003.
  • Now convert this value into cubic feet. To do that, multiply the value by 0.132, and the result should be 17.43 cubic feet.
  • Suppose the pressure difference is 2 kg/cm2. Now, 1 kg/cm2 is equal to 0.968 atm. So for 2 kg/cm2, it will be 1.36 atm (2*0.968 atm). Here, ATM is the atmospheric pressure unit.
  • The time required to refill is 20 seconds.
  • So the calculation will be (V×P×60)÷T

= (17.43×1.36×60) ÷ 20

= 71.1144d

This is the CFM value of your air compressor.

Now, a lot of you might have stumbled in the way, and that is because you arrogantly skipped everything and came down to the calculation process. If you are one of those, please scroll up and read the key things we have mentioned after the intro.


So, this is how to calculate the CFM of an air compressor. I have shown you the easy way to calculate the cfm because I have seen in other websites they have made it complex for the reader to understand.

Hope, you will be benefitted.

You can also check the compressor CFM calculator for faster results. But you might still need the variable we showed in the calculation process.

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