Toll Free: 800-682-6528   |   Local Phone: 252-291-7808   |   Fax:  252-291-1324   |   Email:

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Useful Formulas

Find Motor Pulley Diameter

Flywheel diameter:
Motor RPM:
Compressor RPM:
Motor Pulley:
Motor Pulley Diameter!

You can use just the diameters, but using pitch diameters is more accurate.

Free air if volumetric efficiency (%) and piston displacement are known

Free Air = piston displacement X eff. (%)

To find cubic feet of either compressed air or free air

cu.ft. comp. air X (psig + 14.7) = cu.ft. free air X 14.7

  • cu.ft. comp. air [ Cubic Feet of Compressed Air ]
  • cu.ft. free air [ Cubic Feet of Free Air ]

To convert gallons (tank size) to cubic feet

cu.ft. = gallons X .134

Cubic Feet of free air required to raise a receiver from some pressure to a greater pressure

Tank size in gallons:
Final PSI:
Beginning PSI:
Cubic Ft. Air Required:
Find Air!

If starting from "0" pressure, put "0" in for beginning psi

To Check Operation of Compressor With a Tank

Time how long it takes to pump up the tank from "0" psi to some greater pressure (normal cut off psi)
Calculate the free air using the method above.
Put the time in the following:

Time seconds or CFM:
CFM or Time in seconds:
Find cubic feet per minute or Time! If you enter time in seconds, you will get CFM. If you enter CFM you will get the time to pump up to the final pressure.

Air Cylinders

Total force in lbs. of air cylinder = area of the cylinder diameter (in²) x psi gauge pressure
CFM of Free Air req'd to Operate Air Cylinder (single acting)
= vol. of cyl. (in cu. ft.) x cycles per minute x (gauge psi + 14.7)/14.7
For double acting air cylinder multiple by 2

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Air Compressor Equipment Inc.
P.O. Box 3961, 1901 Baldree Road, Wilson, NC  27895
Toll Free: 800-682-6528  /  Phone: 252-291-7808  /  Fax: 252-291-1324  /  Email:


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