Plenum Cross-Section Calculator
Supply plenum cross-section from air handler CFM.
Enter your values
Enter the air handler cfm in CFM.
Enter the plenum velocity in FPM.
Enter the cabinet width in in.
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.
- Required cross-section area
- 2.333 sq ft
- Plenum height at cabinet width
- 16.8 in
- Round equivalent diameter
- 20.7 in
Quick answer
Supply plenums are sized so velocity leaving the air handler cabinet stays around 500-700 FPM to limit noise right at the source.
Area = CFM / velocity
At a glance
| What it does | Supply plenum cross-section from air handler CFM. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Air handler CFM, Plenum velocity, Cabinet width |
| Main output | Required cross-section area |
| Formula | Area = CFM / velocity |
| Cost | Free — no sign-up, no download |
How to use the Plenum Cross-Section Calculator
- 1Enter the air handler's rated CFM at the design static pressure.
- 2Choose a velocity around 500-700 FPM leaving the cabinet.
- 3Solve for plenum height at the cabinet's width, or vice versa.
Inputs explained
- Air handler CFM(CFM)
- Enter the air handler cfm in CFM.
- Plenum velocity(FPM)
- Enter the plenum velocity in FPM.
- Cabinet width(in)
- Enter the cabinet width in in.
Worked example
Using the values the calculator loads with:
Inputs
- Air handler CFM1400 CFM
- Plenum velocity600 FPM
- Cabinet width20 in
Results
- Required cross-section area2.333 sq ft
- Plenum height at cabinet width16.8 in
- Round equivalent diameter20.7 in
Frequently asked questions
Why size the plenum close to the equipment specially?
It's the highest-velocity point in the system; getting it right prevents excess noise and turbulence right at the unit.
Supply plenum vs return plenum?
Both follow the same approach, though return plenums are often sized a bit larger for lower, quieter velocity.
What do I need to enter into the Plenum Cross-Section Calculator?
Just 3 values: air handler cfm, plenum velocity, cabinet width. 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 Plenum Cross-Section Calculator work out the answer?
Supply plenums are sized so velocity leaving the air handler cabinet stays around 500-700 FPM to limit noise right at the source. It applies the formula Area = CFM / velocity and shows the working so you can check each step by hand.
Is the Plenum Cross-Section 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.