Return Air Duct Sizing Calculator
Central return duct size from system CFM.
Enter your values
ⓘ Live results
Results
- Round return duct diameter
- 22.1 in
- Cross-section area
- 2.667 sq ft
- Rectangular at 20 in height
- 19.2 x 20 in
Quick answer
Return ducts are usually sized for 500-700 FPM, lower than supply trunks, since return noise is harder to attenuate.
Diameter = sqrt(4 x CFM / (velocity x pi)) x 12
How to use the Return Air Duct Sizing Calculator
- 1Total the system's design return CFM, matching supply airflow.
- 2Use 500-700 FPM for the return trunk to limit noise.
- 3Check the rectangular option against your framing cavity depth.
Worked example
Using the values the calculator loads with:
Inputs
- Total return CFM1600 CFM
- Design velocity600 FPM
Results
- Round return duct diameter22.1 in
- Cross-section area2.667 sq ft
- Rectangular at 20 in height19.2 x 20 in
Frequently asked questions
Why size return lower velocity than supply?
Returns are closer to occupied spaces and lack the distance and fittings that attenuate supply noise.
One big return or many small ones?
Multiple smaller returns near each room improve comfort and balance better than a single central return.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.