Jumper Duct Size Calculator
Size a jumper duct to relieve a closed room.
Enter your values
Enter the room supply 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 jumper duct diameter
- 7 in
- Minimum practical size
- 6 in round or 10x6 in rectangular boot
- Two grille terminations needed
- One in room ceiling, one near central return
Quick answer
Jumper ducts are typically sized for 400-500 FPM, similar to a branch return, based on the room's supply CFM.
Diameter = sqrt(4 x CFM / (velocity x pi)) x 12
At a glance
| What it does | Size a jumper duct to relieve a closed room. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Room supply CFM, Design velocity |
| Main output | Round jumper duct diameter |
| Formula | Diameter = sqrt(4 x CFM / (velocity x pi)) x 12 |
| Cost | Free — no sign-up, no download |
How to use the Jumper Duct Size Calculator
- 1Enter the CFM supplied to the closed-door room.
- 2Use about 450 FPM design velocity for a quiet jumper duct.
- 3Terminate the duct with a grille in the room and another opening into the hallway or return plenum.
Inputs explained
- Room supply CFM(CFM)
- Enter the room supply cfm in CFM.
- Design velocity(FPM)
- Enter the design velocity in FPM.
Worked example
Using the values the calculator loads with:
Inputs
- Room supply CFM120 CFM
- Design velocity450 FPM
Results
- Round jumper duct diameter7 in
- Minimum practical size6 in round or 10x6 in rectangular boot
- Two grille terminations neededOne in room ceiling, one near central return
Frequently asked questions
Why not just undercut the door?
Undercuts alone rarely relieve enough airflow for a fully closed, actively supplied room; a jumper duct moves much more air quietly.
Does a jumper duct need a fan?
No, it relies on the pressure difference created by the supply air pressurizing the room.
What do I need to enter into the Jumper Duct Size Calculator?
Just 2 values: room supply 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 Jumper Duct Size Calculator work out the answer?
Jumper ducts are typically sized for 400-500 FPM, similar to a branch return, based on the room's supply CFM. 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 Jumper 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.