Register Size Calculator
Supply register size from CFM and face velocity.
Reviewed by James Whitfield · How we check our maths
Your values
ⓘ Live results
Results
- 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.
The formula
Free area = CFM / face velocity
Worked example
Worked example
Using the values the calculator loads with.
Inputs
- Airflow120 CFM
- Face velocity500 FPM
- Free area fraction75 %
Result
- 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
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.
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.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error