Return Air Duct Sizing Calculator
Central return duct size from system CFM.
Enter your values
Enter the total return cfm in CFM.
Enter the design velocity in FPM.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Central return duct size from system CFM. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Total return CFM, Design velocity |
| Main output | Round return duct diameter |
| Formula | Diameter = sqrt(4 x CFM / (velocity x pi)) x 12 |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Total return CFM(CFM)
- Enter the total return cfm in CFM.
- Design velocity(FPM)
- Enter the design velocity in FPM.
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.
What do I need to enter into the Return Air Duct Sizing Calculator?
Just 2 values: total return cfm, design velocity. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Return Air Duct Sizing Calculator work out the answer?
Return ducts are usually sized for 500-700 FPM, lower than supply trunks, since return noise is harder to attenuate. It applies the formula Diameter = sqrt(4 x CFM / (velocity x pi)) x 12 and shows the working so you can check each step by hand.
Is the Return Air Duct Sizing Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.