Extended Plenum Size Calculator
Size an extended plenum trunk system.
Enter your values
ⓘ Live results
Results
- Plenum size at furnace
- 28.8 x 10 in
- Cross-section area
- 2 sq ft
- Round equivalent
- 19.1 in
- Recommended taper
- Reduce width every 3-4 takeoffs to hold velocity
Quick answer
An extended plenum is usually sized so velocity at the furnace end is 700-900 FPM, tapering airflow demand as branches peel off along its length.
Width = (CFM / velocity x 144) / height
How to use the Extended Plenum Size Calculator
- 1Enter the total furnace or air handler CFM.
- 2Choose an available plenum height that fits the space.
- 3Use the calculated width at the furnace end and taper it down as branches take off air.
Worked example
Using the values the calculator loads with:
Inputs
- Furnace airflow1600 CFM
- Plenum height10 in
- Starting velocity800 FPM
Results
- Plenum size at furnace28.8 x 10 in
- Cross-section area2 sq ft
- Round equivalent19.1 in
- Recommended taperReduce width every 3-4 takeoffs to hold velocity
Frequently asked questions
Extended plenum vs radial?
Extended plenum trunks run a long single duct with branches off the side, while radial systems run individual ducts from a central plenum box.
Does an extended plenum need tapering?
Ideally yes, though many residential systems use a single constant size for simplicity at some efficiency cost.
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.