The CFM of an air compressor is the "cubic feet per minute." This is a measurement of how much air is passing through the air compressor within a single 60-second time span. Calculating the CFM of an air compressor involves finding out a few pieces of information about the way it operates, like how much air the compressor can hold when full and how fast air leaves the device while you're using it.

Divide the total number of gallons of air in your air compressor by the number 7.48. The total number of gallons of air (also called the volume) will be listed on your tank, in its manual and on any packaging materials it came in. This will give you the volume of your air compressor in cubic feet.

Find out how long a normal cycle is for your air compressor. A cycle is the period from the moment the air compressor turns on during use to the moment it turns off during use. While recording this time, also look at the PSI gauge built into the compressor and record the PSI readings at these two points.

Subtract the PSI reading from when the compressor turns off with the PSI reading from when the compressor turns on. This will give you the PSI difference in use during your air compressor's cycle. Divide this number by 14.7 to convert it to the "ATM" unit of measurement.

Multiply the volume of your air compressor in cubic feet by the number of ATM units in one cycle of your air compressor.

Divide the number 60 by the number of seconds in one cycle of your air compressor. Multiply the number you get by the number you got in Step 4 to find the CFM of your air compressor.