ACCA Manual D • 0.08 in. wg Friction500 CFM • 1.25 TonsWhisper Quiet (<700 FPM)
12-Inch Flex Duct CFM Capacity & Sizing Guide
Calculated duct sizing specifications for 500 CFM of conditioned airflow. Sized via the Equal Friction Method and Huebscher equivalent rectangular equation to maintain static pressure and eliminate air rush noise.
Round Duct Size12" RoundSpiral / Snaplock Pipe
Air Velocity637 FPMFeet Per Minute
Cooling Tonnage1.25 Tons@ 400 CFM / Ton
Register / Grille14x20Face Dimension
Equivalent Rectangular Duct Sizing Schedule
Exact rectangular sheet metal dimensions offering equivalent airflow resistance to a 12" round pipe
| Height (Depth) | Width | Cross Area | Air Velocity | Aspect Ratio | SMACNA Status |
|---|---|---|---|---|---|
| 10" Deep | 16" Wide | 160 sq in | 450 FPM | 1.6:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:14x20 Return Grille / Sub-Trunk Transition
Maximum Target Velocity:500–700 FPM at register face
Diffuser Throw:Spread toward outside walls
Return Air Intake Rules
Return Velocity Target:<450 FPM through filter face
Filter Surface Requirement:Minimum 2 sq ft per Ton
Return Friction Target:0.05 – 0.08 in. wg / 100 ft
HVAC Sheet Metal & Installation Directives
- A 12-inch flex duct moves 500 CFM, equivalent to 1.25 tons of AC airflow.
- Frequently used for dedicated return air drops from hallway ceilings in single-story attic installations.
Frequently Asked Duct Sizing Questions
What is the max CFM for a 12-inch flex duct?
A 12-inch flex duct is rated for 480 to 580 CFM at standard residential friction rates.