Floor Register Size Calculator
Size a floor supply register.
Enter your values
Enter the register cfm 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
- 0.222 sq ft
- Gross opening size
- 0.317 sq ft
- Approx size
- 6.8 x 6.8 in
Quick answer
Floor registers are commonly sized for 400-500 FPM face velocity since air exits close to occupants and must stay gentle.
Free area = CFM / face velocity
At a glance
| What it does | Size a floor supply register. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Register CFM, 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 Floor Register Size Calculator
- 1Enter the floor register's design CFM.
- 2Use a lower velocity, 400-500 FPM, so the jet does not blow directly on feet.
- 3Size the opening from the free-area fraction and compare it against standard floor register stock sizes.
Inputs explained
- Register CFM(CFM)
- Enter the register cfm 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
- Register CFM100 CFM
- Face velocity450 FPM
- Free area fraction70 %
Results
- Required free area0.222 sq ft
- Gross opening size0.317 sq ft
- Approx size6.8 x 6.8 in
Frequently asked questions
Why lower velocity for floor registers?
They are close to occupants standing or sitting nearby, so a gentler jet avoids drafts.
Are floor registers good for cooling?
Cold air naturally sinks, so floor supply can struggle to mix in cooling mode compared with ceiling or high sidewall supply.
What do I need to enter into the Floor Register Size Calculator?
Just 3 values: register cfm, 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 Floor Register Size Calculator work out the answer?
Floor registers are commonly sized for 400-500 FPM face velocity since air exits close to occupants and must stay gentle. It applies the formula Free area = CFM / face velocity and shows the working so you can check each step by hand.
Is the Floor 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.