Return Air Duct Size Calculator
Return grille and duct size for your airflow.
Enter your values
ⓘ Live results
Results
- Return duct area
- 288 sq in
- Round duct diameter
- 19.1 in
- Grille free area
- 432 sq in
- Nominal grille size
- 24 x 24 in (75% free area)
Quick answer
Return air should travel slower than supply — 500–700 FPM in duct and about 300–500 FPM through the grille face.
area = CFM / velocity
How to use the Return Air Duct Size Calculator
- 1Enter the system airflow, typically 400 CFM per ton.
- 2Keep duct velocity at 500–700 FPM for quiet operation.
- 3Size the grille on free area, not the nominal size.
Worked example
Using the values the calculator loads with:
Inputs
- System airflow1200 CFM
- Duct velocity600 FPM
- Grille face velocity400 FPM
Results
- Return duct area288 sq in
- Round duct diameter19.1 in
- Grille free area432 sq in
- Nominal grille size24 x 24 in (75% free area)
Frequently asked questions
Why are returns usually too small?
Installers size returns to fit the joist bay rather than the airflow, which is the top cause of high static pressure.
Can I add a second return?
Yes — two smaller returns are quieter and more effective than one oversized grille.
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.