# If An Exhaust Fan Revolves 3000 Rpm, What Is The CFM Air Flow?

I need some info and I can answer this relatively easy or here is the formula if you want to try it yourself.

Air velocity is calculated by dividing the volumetric flow rate by the cross-sectional area of the air flow passage.
Velocity (LFM) = Volume (CFM) / area (ft²)

Velocity (m/s) = Volume (m³/s) / area (m²)

Air flow is usually provide by one or more fans. Although most fans are normally rated and compared at their free air delivery at zero back pressure, nearly all applications have flow resistance so the fans act against some back pressure. For axial fans the volumetric flow rate can be derated to about 60% to 80% of free air delivery to anticipate typical levels of backpressure.

Example: The free air volume delivered by a fan is given as 80 CFM. The area of the air flow passage is 6 inches by 6 inches or 36 in² or 0.25 ft².

Or you can give me the measurements of the fan itself, number of blades etc. Hope this helps.
thanked the writer.
Anonymous commented

Box is 23 1/2" in width X 20" in depth X 19 3/4" in fight. The fan is 5 1/2" in diameter. There are four blades to the fan.

I am trying to build a Airbrush Paint Spray booth, and the above are the measurements of that box. I am contemplating in using 6" diameter duct work, since the fan is 5 1/2" in diameter. If you think that 6" diameter duct work is sufficient, or please let me know if I need a larger diameter duct work.