Return Air Grille Size Calculator
Return grille size from return CFM.
Enter your values
ⓘ Live results
Results
- Required free area
- 1 sq ft
- Gross grille area
- 1.429 sq ft
- Approx square size
- 14.3 x 14.3 in
- Round duct equivalent
- 10.2 in at 700 FPM
Quick answer
Return grilles are usually sized for 300-500 FPM face velocity through their free area, lower than supply registers to cut noise.
Free area = return CFM / face velocity
How to use the Return Air Grille Size Calculator
- 1Enter the total return airflow the grille must handle.
- 2Use 300-400 FPM for central returns near living areas, up to 500 FPM elsewhere.
- 3Divide by the grille's free-area fraction to get gross size.
Worked example
Using the values the calculator loads with:
Inputs
- Return airflow400 CFM
- Face velocity400 FPM
- Free area fraction70 %
Results
- Required free area1 sq ft
- Gross grille area1.429 sq ft
- Approx square size14.3 x 14.3 in
- Round duct equivalent10.2 in at 700 FPM
Frequently asked questions
Should returns be bigger than supplies?
Yes, returns are usually oversized relative to supply air to keep velocity and noise low.
One return or several?
Central return systems use one large grille; better designs add returns in each closed-door room.
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.