Free Area Register Calculator
Percent free area and true velocity of a register.
Enter your values
ⓘ Live results
Results
- Percent free area
- 75%
- True face velocity
- 800 FPM
- Nominal velocity (gross)
- 600 FPM
Quick answer
Percent free area is the open airflow area divided by the overall grille frame area, typically 60-85% for standard registers.
Free area % = open area / gross area x 100
How to use the Free Area Register Calculator
- 1Measure or find the manufacturer's open area and the overall grille size.
- 2Enter the CFM the register is passing.
- 3Compare true velocity through the free area against the 400-600 FPM comfort range.
Worked example
Using the values the calculator loads with:
Inputs
- Open (free) area18 sq in
- Gross frame area24 sq in
- Airflow100 CFM
Results
- Percent free area75%
- True face velocity800 FPM
- Nominal velocity (gross)600 FPM
Frequently asked questions
Why is true velocity higher than nominal?
Because it uses only the open slot area, not the full frame, so it is always higher.
Typical free area for bar registers?
Around 60-75%, while linear slot diffusers can exceed 80%.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.