Extended Plenum Size Calculator
Size an extended plenum trunk system.
Enter your values
Enter the furnace airflow in CFM.
Enter the plenum height in in.
Enter the starting 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.
- 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
At a glance
| What it does | Size an extended plenum trunk system. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Furnace airflow, Plenum height, Starting velocity |
| Main output | Plenum size at furnace |
| Formula | Width = (CFM / velocity x 144) / height |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Furnace airflow(CFM)
- Enter the furnace airflow in CFM.
- Plenum height(in)
- Enter the plenum height in in.
- Starting velocity(FPM)
- Enter the starting velocity in FPM.
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.
What do I need to enter into the Extended Plenum Size Calculator?
Just 3 values: furnace airflow, plenum height, starting 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 Extended Plenum Size Calculator work out the 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. It applies the formula Width = (CFM / velocity x 144) / height and shows the working so you can check each step by hand.
Is the Extended Plenum 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.