## How do you calculate CFM from RPM for fans?

Define the terms in the general centrifugal fan formula and rearrange them to use the formula to solve for the higher air flow: **CFM 1/CFM 2 = RPM 1/RPM 2 x (D1/D2)^3 (cubed)**. The (D1/D2)^3 is 1, since the impeller diameter stays the same, so CFM 2 = CFM 1 x (RPM 2/RPM 1).

## How is CFM calculated?

To calculate Air Flow in Cubic Feet per Minute (CFM), **determine the Flow Velocity in feet per minute, then multiply this figure by the Duct Cross Sectional Area**.