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APPENDIX C – AIRFLOW VOLUME CALCULATIONS
To calculate cubic feet per minute (CFM) from a measured air velocity (FPM),
you need the calculated cross-sectional area of the air flow stream:
Volume Flow (CFM) = Velocity (FPM) X Area (sq ft).
In a rectangular duct this cross-sectional area equals the Width times the Height.
W x H=A (cross-sectional area)
In a circular duct this cross-section area equals the radius squared times
π
(3.14).
R x R x 3.14=A (cross-sectional area)
To convert an area calculated in square inches to an area calculated in square
feet (which is required for the Volume Flow equation above) divide by 144:
(area in sq in.)/144 = (area in sq ft.).
Example:
The air duct is rectangular; the width is 24 in., and the height
is 12 in. The air velocity reading through the duct is 450 FPM.
Calculate the Volume Flow.
Step 1:
Cross-sectional area = 24 in. x 12 in. = 288 sq in.
Step 2:
288 sq in /144 = 2 sq ft.
Step 3:
Volume flow = Air Velocity x Area, therefore,
Volume flow rate = 450 FPM x 2 sq ft. = 900 CFM.