How to find out how many CFM your compressor delivers

Posted: June 6, 2012 in Uncategorized
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The most used, most needed and most valuable Compressed Air Formulas

Do you have access to the most used, most needed and most valuable Compressed Air Formulas? Here’s one of the most used formulas you should keep on hand…
How to find how many CFM your air compressor delivers.

Follow these steps to find out how many
CFM Your Air Compressor actually delivers

1. STOP the compressor unit

2. CLOSE the outlet valve on the tank/air receiver

3. DRAIN the condensate from air receiver until there is 0 PSIG -then close the drain valve

4. NOTE THE TIME– in minutes & seconds (Best to write it down.) Then START THE UNIT.
When the compressor unit stops and unloads – then NOTE THE TIME again – in minutes & seconds. Convert the minutes into seconds and then total the number of seconds it takes between START and STOP/UNLOAD.

5. NOTE the GUAGE PSIG reading

6. NOTE the Air Receiver/Tank GALLON SIZE


TANK GALLONS x .536* x PSIG divided by SECONDS

*.536 is a formula factor for the unknown that works to give you the CFM delivery.

You have an 80 gallon tank, your total start to stop/unload time was 3 minutes and 9 seconds.
Change the minutes to seconds timed (60 x 3= 180 seconds plus 9 seconds totals 189).  You will use the total number of seconds189 and the noted 175 PSIG within the formula as shown below:

80 multiplied by .536 = 42.88
42.88 multiplied by 175 (example PSIG) = 7504.00
7504.00 divided by 189 (total seconds)= 39.70 CFM delivered

You now know that your air compressor is delivering 39.70 CFM

Your Response to this evaluation should be to compare this number with what your air compressor manufacturer says your CFM should be and evaluate how efficiently your compressor is running.


Compressor Terms you should know:

Cubic Feet Per Minute (cfm) – Volumetric air flow rate.

“psig” means pounds per square inch, GAGE pressure. Gage pressure is the absolute pressure of something, with the atmospheric pressure subtracted. In practice, when someone gives a pressure in just “psi” they probably mean gage pressure. If they mean absolute, they should be using “psia.”

Gauge Pressure – The pressure determined by most instruments and gauges, usually expressed in psig. Barometric pressure must be considered to obtain true or absolute pressure

Load Time – Time period from when a compressor loads until it unloads.

Unload – (No load) Compressor operation in which no air is delivered due to the intake being closed or modified not to allow inlet air to be trapped.

Receiver – A vessel or tank used for storage of gas under pressure. In a large compressed air system there may be primary and secondary receivers.

Demand – Flow of air at specific conditions required at a point or by the overall facility.

Tommy McGuire

McGuire Air Compressors, Inc.
“Real People with Real Compressor Experience”




  1. saravanan says:

    why we want to multiply the constant 0.538

  2. Raman says:

    excellent explanation hats off to you. please tell us also how to select air compressor capacity (CFM) for unknown air consumption.

  3. Brian says:

    This is truly a great read for me. I have bookmarked it and I am looking forward to reading new articles. Keep up the good work!.

  4. Charles Mann says:


  5. Arjun kshirsagar says:

    thanks for making calculation easy…

  6. Wayn Goodman says:

    what is the .536 constant derived from?

    • The .536 is an average arrived from much use of the long formula for CFM delivery. The long engineering formula takes into account the ambient temperature, barometric and absolute pressure and humidity. After many years of working this longer formula, I discovered a numeric average that works. This average (.536) seems to work out to less than 1% variance from the long, complicated engineering formula. That’s where it comes from.

  7. sengottaiyan sengottaiyan says:

    Sir pls please explain PSIG. I don’t understand that

  8. Your Article helped a lot …..Thanks

  9. md salman says:

    it really helpful for the beginners who like to know about the air-compressor …..good work…

  10. Arpit says:

    really very nicely explained, thanks a lot

  11. Manvendra Choubey says:

    Useful artical and explaning method is good

  12. ankur says:

    thanks for share knowledge

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