Return Air Duct Size Calculator
Return grille and duct size for your airflow.
Enter your values
Enter the system airflow in CFM.
Enter the duct velocity in FPM.
Enter the grille face 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.
- 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
At a glance
| What it does | Return grille and duct size for your airflow. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | System airflow, Duct velocity, Grille face velocity |
| Main output | Return duct area |
| Formula | area = CFM / velocity |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- System airflow(CFM)
- Enter the system airflow in CFM.
- Duct velocity(FPM)
- Enter the duct velocity in FPM.
- Grille face velocity(FPM)
- Enter the grille face velocity in FPM.
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.
What do I need to enter into the Return Air Duct Size Calculator?
Just 3 values: system airflow, duct velocity, grille face 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 Size Calculator work out the answer?
Return air should travel slower than supply — 500–700 FPM in duct and about 300–500 FPM through the grille face. It applies the formula area = CFM / velocity and shows the working so you can check each step by hand.
Is the Return Air Duct Size 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.