Duct design
Rectangular Duct Size Calculator
Calculate a preliminary rectangular duct width and height.
Calculator
How to use
Set airflow, target velocity, and a practical width-to-height ratio. The calculator derives dimensions with the same face area.
Formula
Area = 144 × CFM ÷ FPM; H = √(Area ÷ ratio); W = ratio × H
Variables
Area is in square inches; ratio is width divided by height.
Example
800 CFM at 700 FPM with a 2:1 ratio produces approximately 21.6 × 10.8 in.
Result interpretation
Verify the selected duct's equivalent diameter and pressure loss; a very high aspect ratio can increase friction and fabrication constraints.
FAQ
Can I use any aspect ratio?
Physical space and design criteria govern the choice. Check the final selection against your duct design method.