Cubic Feet of Air in Rectangular Duct:

CFM= 〖Area of duct ft〗^(2 ) x FPM

CFM = Cubic Feet Per Minute
Area in square feet = Square feet of the duct if you measure the duct dimensions in inches and you have square inches then divided by 144 to get Square Feet.
FPM = Feet Per Minute

Cubic Feet of Air in Round Duct:

CFM= 〖Area of duct ft〗^(2 ) x FPM

CFM = Cubic Feet Per Minute
Area in square feet = Square feet of the duct if you measure the duct dimensions in inches and you have square inches then divided by 144 to get Square Feet.

Circle Area in 〖FT〗^2=(3.14 x 〖Radius〗^2)/144"

FPM = Feet Per Minute


CFM out of a Round Grille you can’t access with a hood = Area x Average Velocity x Correction Factor

CFM Grille= 〖Area of duct ft〗^(2 ) x Average FPM x Correction Factor

CFM Grille = Cubic Feet Per Minute
Circle Area in 〖FT〗^2=(3.14 x 〖Radius〗^2)/144"

Example: You have an 18”
Step 1: Circle Area in 〖FT〗^2=(3.14 x 9^2)/144"

Step 2: Circle Area in 〖FT〗^2=(3.14 x 81)/144"

Step 3: Circle Area in 〖FT〗^2=244/144"

Step 4: Circle Area in 〖FT〗^2=1.76

Average FPM = Feet Per Minute averaged between multiple readings/traversing from the grille.

Traverse Correction Factor (used for anemometers)

Correction Factor= (Hood Measurement on another similar Grille)/(Anemometer Traverse Measurment (with anemometer 1" from grille))

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