Calculating CFM Too

This is page two of calculating CFM. Here is page one.

Page two of calculating CFM begins with a question and comments from Greg about how long his compressed air will last.

Hi Bill:

Ok, I have it. Here's the basics --- the air flow and CFM are based upon three things:

  • 1: The larger the tank, the greater the supply of air (longer supply of CFM for a more gradual drop in pressure). This is the "capacitor" or "flywheel" in the system. However, this is only the "time" relationship. If you know your pressure and just want CFM, you don't care about the tank size.
  • 2: Separate the Tank from the compressor (ie, compressor is off). The relationship between CFM and PSI is just linear. Ie, if you charge to 120 psi, and pull the electrical plug....if you get 8 CFM at 120 psi, you will get 4 CFM at 60 PSI. The pressure just "pushes" the air out, and with half the "push" you get half the air flow.
  • 3: The motor and compressor. Assume the compressor to be 100% efficient, and the motor to be 80% efficient. A general rule of thumb is that a 5hp motor can produce 10 CFM at 100psi. The BEST way to determine your compressor-only CFM vs PSI is to know the steady state running current of your motor. Ie, my old 2hp 20 gall compressor pulls about 6.5 amps at 240VAC. Thats a power pull of 1560 watts. At about 746 watts per HP, that's 2.1 HP. 80% efficiency brings it to 1.7 true HP for compressing air. Going through a bunch of math, the flows I get for my compressor at 80/70/60/50/40/30 psi are: 4.9/5.6/6.5/7.8/9.7/12.9 CFM respectively.

Then, what you do is combine 2 and 3 above for a graph. More basically, if I know I need to blow out my sprinkler system with 50 PSI and 20 GPM, that's 50 PSI and about 3 CFM. If my air reservoir is being depleated and the compressor turns on, I will WORST CASE still get 7.8 GPM at 50 PSI.

Ideally, one would like a spreadsheet that gave CFM per PSI based on true HP. This would not include a third axis (time) which would include bleed-out based on tank size.

Best Regards,


Thanks Greg. I appreciate the contribution to this seemingly increasingly complex issue which is important to all air compressor users.