ACCA Manual D • 0.08 in. wg Friction160 CFM • 0.4 TonsWhisper Quiet (<700 FPM)
8-Inch Flex Duct CFM Capacity & Sizing Guide
Calculated duct sizing specifications for 160 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 Size8" RoundSpiral / Snaplock Pipe
Air Velocity458 FPMFeet Per Minute
Cooling Tonnage0.4 Tons@ 400 CFM / Ton
Register / Grille6x12Face Dimension
Equivalent Rectangular Duct Sizing Schedule
Exact rectangular sheet metal dimensions offering equivalent airflow resistance to a 8" round pipe
| Height (Depth) | Width | Cross Area | Air Velocity | Aspect Ratio | SMACNA Status |
|---|---|---|---|---|---|
| 6" Deep | 10" Wide | 60 sq in | 384 FPM | 1.7:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:6x12 or 4x14 Register Boot
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
- An 8-inch flexible duct comfortably delivers 160 to 200 CFM, ideal for supplying larger master suites and open kitchens.
- Support flex duct with minimum 1.5-inch wide hanging straps spaced no more than 4 feet apart.
Frequently Asked Duct Sizing Questions
How much CFM does an 8-inch flex duct move?
An 8-inch flex duct moves approximately 160 CFM at 0.08 in. wg friction rate, and up to 210 CFM at 0.10 in. wg.