How To Calculate CFMs

A clogged or rusty fan will work far below calculated expectations.
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Your fan's expected blow rate can be calculated from the amount of power it consumes and the pressure at which it works. The calculation will determine the fan's CFM value -- meaning how many cubic feet of air it displaces each minute. This value is essential to know when designing a ventilation system or an industrial exhaust system. If a fan fails to meet an area's ventilation or exhaust needs, you will need to select a different fan or install multiple units.

    Convert the fan's stated power rating from kilowatts to horsepower by multiplying by 1.34. If the fan works, for instance, at 6 kW, then 6 x 1.34 = 8.04.

    Convert the fan's rated pressure, which manufacturer documentation specifies, from pounds per square inch to feet of water by dividing by 0.433. If the fan works, for instance, producing 0.1 psi: 0.1 / 0.433 = 0.23 feet of water.

    Divide the horsepower by the fan's pressure: 8.04 / 0.23 = 34.96.

    Multiply the result by 530, a conversion constant: 34.96 x 530 = 18,528.8, or approximately 18,500. This is the number of cubic feet of air the fan should displace each minute.