Register Size Calculator
Supply register size from CFM and face velocity.
Enter your values
Enter the airflow in CFM.
Enter the face 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
- 0.24 sq ft
- Gross opening area
- 0.32 sq ft
- Approx square opening
- 6.8 x 6.8 in
- Round equivalent dia
- 6.6 in
Quick answer
Registers are typically sized for 400-600 FPM face velocity through their free area to avoid noise and drafts.
Free area = CFM / face velocity
At a glance
| What it does | Supply register size from CFM and face velocity. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Airflow, Face velocity, Free area fraction |
| Main output | Required free area |
| Formula | Free area = CFM / face velocity |
| Cost | Free — no sign-up, no download |
How to use the Register Size Calculator
- 1Enter the design CFM for the room.
- 2Use 400-500 FPM for living spaces, up to 600-750 for less critical areas.
- 3Divide by the register's published free-area fraction to get gross size.
Inputs explained
- Airflow(CFM)
- Enter the airflow in CFM.
- Face velocity(FPM)
- Enter the face velocity in FPM.
- Free area fraction(%)
- Enter the free area fraction in %.
Worked example
Using the values the calculator loads with:
Inputs
- Airflow120 CFM
- Face velocity500 FPM
- Free area fraction75 %
Results
- Required free area0.24 sq ft
- Gross opening area0.32 sq ft
- Approx square opening6.8 x 6.8 in
- Round equivalent dia6.6 in
Frequently asked questions
Why does free area matter?
Grille bars and louvers block part of the opening, so the actual airflow area is smaller than the frame size.
What if velocity is too high?
High face velocity causes whistling and drafts; drop it toward 400 FPM in quiet rooms.
What do I need to enter into the Register Size Calculator?
Just 3 values: airflow, face 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 Register Size Calculator work out the answer?
Registers are typically sized for 400-600 FPM face velocity through their free area to avoid noise and drafts. It applies the formula Free area = CFM / face velocity and shows the working so you can check each step by hand.
Is the Register 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.