Duct design
Rectangular Duct Size Calculator
Calculate preliminary rectangular width and height from assigned CFM, target velocity, and a chosen aspect ratio.
Calculate rectangular dimensions
The aspect ratio is a layout choice; it is not a friction or noise approval.
How to use
- Assign CFM to the specific duct run.
- Enter a project-selected target velocity.
- Enter a practical width-to-height ratio and use the output as a theoretical starting size.
Formula
Height = √(area ÷ ratio); Width = ratio × height
Variables
The aspect ratio is width divided by height. It redistributes the same calculated face area; it does not make different shapes behave identically in a real duct system.
Example
At 800 CFM and 700 FPM, required face area is 164.6 in². With a 2:1 ratio, the theoretical dimensions are approximately 18.1 × 9.1 in.
Result interpretation
Select a practical available duct and recalculate its actual area and velocity. Then review equivalent diameter, material, fittings, transitions, straight length, terminals, available fan pressure, noise, and balancing. High aspect ratios can add fabrication constraints and change resistance.
Continue the geometry check
Use the Equivalent Duct Diameter Calculator for an equal-friction approximation, then use the Duct Friction Loss Calculator only with documented air properties and friction inputs.
Frequently asked questions
Can I use any aspect ratio?
Physical space and design criteria govern the choice. Check the selected shape with the project duct design method and complete pressure path.
Does equal face area mean equal pressure loss?
No. Geometry, roughness, fittings, and equivalent diameter influence resistance. Equal area is only the starting geometry relationship.