Transfer Grille Size Calculator
Size a transfer grille for a closed-door room.
Enter your values
Enter the room supply cfm in CFM.
Enter the transfer velocity in FPM.
Enter the free area fraction in %.
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.
- Required free area
- 0.4 sq ft
- Gross grille size
- 0.571 sq ft
- Approx square size
- 9.1 x 9.1 in
Quick answer
Transfer grilles are sized for low velocity, typically 200-300 FPM, since low noise matters more than compact size in a wall opening.
Free area = supply CFM / transfer velocity
At a glance
| What it does | Size a transfer grille for a closed-door room. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Room supply CFM, Transfer velocity, Free area fraction |
| Main output | Required free area |
| Formula | Free area = supply CFM / transfer velocity |
| Cost | Free — no sign-up, no download |
How to use the Transfer Grille Size Calculator
- 1Enter the CFM being supplied to the closed-door room.
- 2Use 200-300 FPM for a quiet transfer path.
- 3Size the grille's gross opening from its free-area fraction.
Inputs explained
- Room supply CFM(CFM)
- Enter the room supply cfm in CFM.
- Transfer velocity(FPM)
- Enter the transfer velocity in FPM.
- Free area fraction(%)
- Enter the free area fraction in %.
Worked example
Using the values the calculator loads with:
Inputs
- Room supply CFM100 CFM
- Transfer velocity250 FPM
- Free area fraction70 %
Results
- Required free area0.4 sq ft
- Gross grille size0.571 sq ft
- Approx square size9.1 x 9.1 in
Frequently asked questions
Transfer grille vs jumper duct?
A transfer grille passes air directly through the wall, while a jumper duct routes it over the top of the wall through the attic or ceiling space.
Do transfer grilles block sound?
Standard transfer grilles pass sound easily; acoustically lined versions or offset ducted transfers reduce noise transfer.
What do I need to enter into the Transfer Grille Size Calculator?
Just 3 values: room supply cfm, transfer velocity, free area fraction. 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 Transfer Grille Size Calculator work out the answer?
Transfer grilles are sized for low velocity, typically 200-300 FPM, since low noise matters more than compact size in a wall opening. It applies the formula Free area = supply CFM / transfer velocity and shows the working so you can check each step by hand.
Is the Transfer Grille 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.