ACCA Manual D • 0.08 in. wg Friction75 CFM • 0.187 TonsWhisper Quiet (<700 FPM)
6-Inch Flex Duct CFM Capacity & Sizing Guide
Calculated duct sizing specifications for 75 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 Size6" RoundSpiral / Snaplock Pipe
Air Velocity382 FPMFeet Per Minute
Cooling Tonnage0.187 Tons@ 400 CFM / Ton
Register / Grille4x10Face Dimension
Equivalent Rectangular Duct Sizing Schedule
Exact rectangular sheet metal dimensions offering equivalent airflow resistance to a 6" round pipe
| Height (Depth) | Width | Cross Area | Air Velocity | Aspect Ratio | SMACNA Status |
|---|---|---|---|---|---|
| 4" Deep | 8" Wide | 32 sq in | 338 FPM | 2.0:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:4x10 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
- A 6-inch flex duct reliably moves 75 CFM at 0.08 in. wg friction rate, or up to 90 CFM at 0.10 in. wg.
- Flexible duct has 15% to 40% higher friction than rigid sheet metal. Never stretch flex duct around sharp joists.
Frequently Asked Duct Sizing Questions
How many CFM can a 6-inch flex duct handle?
A 6-inch flex duct handles 75 CFM for whisper-quiet bedroom operation, with a maximum threshold of 100 CFM before noticeable air rush noise occurs.