Filter Grille Size Calculator
Size a return filter grille for low pressure drop.
Enter your values
Enter the return airflow in CFM.
Enter the filter 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.
- Required filter area
- 2.67 sq ft
- Approx square filter size
- 19.6 x 19.6 in
- 20x25 filters needed
- 0.8 filters
- 20x20 filters needed
- 1 filters
Quick answer
Filter grilles are sized for a lower face velocity than plain return grilles, typically 250-350 FPM, because the filter media adds resistance.
Filter area = return CFM / face velocity
At a glance
| What it does | Size a return filter grille for low pressure drop. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Return airflow, Filter face velocity |
| Main output | Required filter area |
| Formula | Filter area = return CFM / face velocity |
| Cost | Free — no sign-up, no download |
How to use the Filter Grille Size Calculator
- 1Enter the total return CFM that will pass through the filter grille.
- 2Use 250-300 FPM face velocity for a MERV 8-13 filter to control pressure drop.
- 3Compare against common filter sizes to see how many grilles or how large a single grille needs to be.
Inputs explained
- Return airflow(CFM)
- Enter the return airflow in CFM.
- Filter face velocity(FPM)
- Enter the filter face velocity in FPM.
Worked example
Using the values the calculator loads with:
Inputs
- Return airflow800 CFM
- Filter face velocity300 FPM
Results
- Required filter area2.67 sq ft
- Approx square filter size19.6 x 19.6 in
- 20x25 filters needed0.8 filters
- 20x20 filters needed1 filters
Frequently asked questions
Why lower velocity than a plain grille?
Filter media adds resistance, so lower face velocity keeps total static pressure and noise reasonable.
What if my filter grille is undersized?
It raises static pressure, cuts airflow and shortens filter life due to higher loading per square inch.
What do I need to enter into the Filter Grille Size Calculator?
Just 2 values: return airflow, filter 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 Filter Grille Size Calculator work out the answer?
Filter grilles are sized for a lower face velocity than plain return grilles, typically 250-350 FPM, because the filter media adds resistance. It applies the formula Filter area = return CFM / face velocity and shows the working so you can check each step by hand.
Is the Filter 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.