Air-duct calculators are very useful tools to use when sizing duct. You can use these calculations to fi nd the proper duct size based on the CFM or FPM. The calculations also make it very easy to fi nd the square equivalent of a round duct or the round equivalent of a square duct.
Air-Duct Sizing Formulas
Use the following formula to size a duct for the proper CFM:
CFM = Area * FPM
Example: 12 * 12 duct at 750 FPM; 12 * 12 = 1 sq. ft.
1 sq ft * 750 FPM = 750 CFM
Use the following formula to change the shape of a duct from round to square:
Area of a circle = πr²
Area / length = width
Example: Convert a 12″ round duct to a 10″ deep square or
Area of a 12 IN diameter circle = π6² = 113 sq. IN (rounded)
113 / 10 = 11.3
A 12″ round duct = 10 * 11.3 rectangular duct. Use a 10 * 12
Use the following formula to change the shape of a duct from square to round:
2(square root [area / π]) = diameter
Example: Convert a 10 * 10 square duct to an equivalent round
Area = 10 * 10 = 100 sq. IN
2(square root [100/3.14159]) = 2(square root [31.83])
= 2(5.6418) 5 11.28
A 10 * 10 square duct = 11.28″ round duct. Use a standard
12″ diameter duct.
Duct Sizing Guidelines
Calculate the area for square or rectangular using (length * width).
Calculate the area for round using (π * radius² ).
When calculating duct sizes, remember that ducts are not made in 11.5″ round or 12.375″ * 14.24″ rectangular as a standard, so you need to round to the nearest even number for a size, resulting in a 12″ round and a 12 * 14 rectangular duct.
Whenever you change the size of a duct from the size indicated in the engineer’s design, it should be with the intention of improving the constructability. Whenever possible, it is advisable to use the engineer’s design unless you are unable to because of field conditions. If field conditions do not permit an engineered size, then you need to write a request for information (RFI) to inform the engineer about the change being made.
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