Branch Duct Size Calculator
Branch duct size from register CFM.
Enter your values
Enter the branch cfm in CFM.
Enter the design velocity in FPM.
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.
- Round branch diameter
- 5.5 in
- Flex duct recommended size
- 6 in nominal
- Cross-section area
- 0.167 sq ft
Quick answer
Branch ducts feeding a single register are typically sized for 500-700 FPM to keep noise and friction low.
Diameter = sqrt(4 x CFM / (velocity x pi)) x 12
At a glance
| What it does | Branch duct size from register CFM. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Branch CFM, Design velocity |
| Main output | Round branch diameter |
| Formula | Diameter = sqrt(4 x CFM / (velocity x pi)) x 12 |
| Cost | Free — no sign-up, no download |
How to use the Branch Duct Size Calculator
- 1Enter the CFM the branch must deliver to its register.
- 2Use 500-600 FPM for quiet rooms, up to 700 FPM elsewhere.
- 3Round up to the nearest available flex or rigid duct size.
Inputs explained
- Branch CFM(CFM)
- Enter the branch cfm in CFM.
- Design velocity(FPM)
- Enter the design velocity in FPM.
Worked example
Using the values the calculator loads with:
Inputs
- Branch CFM100 CFM
- Design velocity600 FPM
Results
- Round branch diameter5.5 in
- Flex duct recommended size6 in nominal
- Cross-section area0.167 sq ft
Frequently asked questions
What size is typical for a bedroom register?
Most 80-120 CFM branches use 6 inch round duct.
Should I round up or down?
Always round up — undersized branches raise velocity, noise and pressure drop.
What do I need to enter into the Branch Duct Size Calculator?
Just 2 values: branch cfm, design velocity. 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 Branch Duct Size Calculator work out the answer?
Branch ducts feeding a single register are typically sized for 500-700 FPM to keep noise and friction low. It applies the formula Diameter = sqrt(4 x CFM / (velocity x pi)) x 12 and shows the working so you can check each step by hand.
Is the Branch Duct 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.