Jumper Duct Size Calculator
Size a jumper duct to relieve a closed room.
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ⓘ Live results
Results
- 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
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.
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.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.