Duct Takeoff Size Calculator
Size a branch takeoff fitting from a trunk.
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.
- Calculated diameter
- 5.2 in
- Nominal takeoff size
- 6 in round
- Recommended fitting
- Standard round takeoff
Quick answer
Takeoff fittings are sized to match the branch duct diameter feeding a register, typically 4 to 8 inches for residential branches.
Takeoff diameter = branch duct diameter
At a glance
| What it does | Size a branch takeoff fitting from a trunk. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Branch CFM, Design velocity |
| Main output | Calculated diameter |
| Formula | Takeoff diameter = branch duct diameter |
| Cost | Free — no sign-up, no download |
How to use the Duct Takeoff Size Calculator
- 1Enter the branch CFM the takeoff must supply.
- 2Size for the same velocity used for the branch duct itself.
- 3Round up to the nearest standard takeoff size, using a conical fitting for larger branches to cut turbulence.
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 CFM90 CFM
- Design velocity600 FPM
Results
- Calculated diameter5.2 in
- Nominal takeoff size6 in round
- Recommended fittingStandard round takeoff
Frequently asked questions
Does takeoff style matter?
Yes — straight-cut 90-degree takeoffs create far more turbulence loss than conical or bellmouth takeoffs.
Should the takeoff match the branch exactly?
Yes, a takeoff smaller than its branch duct becomes the bottleneck for that run.
What do I need to enter into the Duct Takeoff 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 Duct Takeoff Size Calculator work out the answer?
Takeoff fittings are sized to match the branch duct diameter feeding a register, typically 4 to 8 inches for residential branches. It applies the formula Takeoff diameter = branch duct diameter and shows the working so you can check each step by hand.
Is the Duct Takeoff 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.