Supply Register Sizing Calculator
How many registers and what size for a room.
Enter your values
Enter the room airflow in CFM.
Enter the face velocity in FPM.
Enter the max cfm per register 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.
- Registers needed
- 2
- CFM per register
- 90
- Free area each
- 22 sq in
- Typical size
- 4 x 10 in nominal
- Throw guidance
- 9 ft approx
Quick answer
Supply registers are sized on face velocity, typically 500–700 FPM, with each register handling 100–150 CFM.
face area = CFM / face velocity
At a glance
| What it does | How many registers and what size for a room. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Room airflow, Face velocity, Max CFM per register |
| Main output | Registers needed |
| Formula | face area = CFM / face velocity |
| Cost | Free — no sign-up, no download |
How to use the Supply Register Sizing Calculator
- 1Enter the airflow the room needs.
- 2Keep face velocity around 600 FPM to avoid whistling.
- 3Split into more registers when one would exceed about 150 CFM.
Inputs explained
- Room airflow(CFM)
- Enter the room airflow in CFM.
- Face velocity(FPM)
- Enter the face velocity in FPM.
- Max CFM per register(CFM)
- Enter the max cfm per register in CFM.
Worked example
Using the values the calculator loads with:
Inputs
- Room airflow180 CFM
- Face velocity600 FPM
- Max CFM per register120 CFM
Results
- Registers needed2
- CFM per register90
- Free area each22 sq in
- Typical size4 x 10 in nominal
- Throw guidance9 ft approx
Frequently asked questions
Why does one vent whistle?
Too much CFM through too little free area — split the flow or fit a larger register.
Where should registers go?
Under windows or along exterior walls so the throw washes the coldest surfaces.
What do I need to enter into the Supply Register Sizing Calculator?
Just 3 values: room airflow, face velocity, max cfm per register. 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 Supply Register Sizing Calculator work out the answer?
Supply registers are sized on face velocity, typically 500–700 FPM, with each register handling 100–150 CFM. It applies the formula face area = CFM / face velocity and shows the working so you can check each step by hand.
Is the Supply Register Sizing 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.