Free Area Register Calculator
Percent free area and true velocity of a register.
Enter your values
Enter the open (free) area in sq in.
Enter the gross frame area in sq in.
Enter the airflow in CFM.
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.
- 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
At a glance
| What it does | Percent free area and true velocity of a register. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Open (free) area, Gross frame area, Airflow |
| Main output | Percent free area |
| Formula | Free area % = open area / gross area x 100 |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Open (free) area(sq in)
- Enter the open (free) area in sq in.
- Gross frame area(sq in)
- Enter the gross frame area in sq in.
- Airflow(CFM)
- Enter the airflow in CFM.
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%.
What do I need to enter into the Free Area Register Calculator?
Just 3 values: open (free) area, gross frame area, airflow. 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 Free Area Register Calculator work out the answer?
Percent free area is the open airflow area divided by the overall grille frame area, typically 60-85% for standard registers. It applies the formula Free area % = open area / gross area x 100 and shows the working so you can check each step by hand.
Is the Free Area Register 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.