ACCA Manual D • 0.08 in. wg Friction250 CFM • 0.625 TonsWhisper Quiet (<700 FPM)
250 CFM Duct Size: Kitchen & Family Room Open Area
Calculated duct sizing specifications for 250 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 Velocity716 FPMFeet Per Minute
Cooling Tonnage0.625 Tons@ 400 CFM / Ton
Register / Grille6x14Face 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 | 12" Wide | 72 sq in | 500 FPM | 2.0:1 | Optimal (<2:1) |
| 8" Deep | 10" Wide | 80 sq in | 450 FPM | 1.3:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:6x14 or 8x12 Register
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 round duct is the standard size for 250 CFM, keeping velocity well below 750 FPM.
- In kitchens, locate supply registers away from gas cooktops and exhaust hoods to prevent disturbing burner flames.
Frequently Asked Duct Sizing Questions
How much CFM can an 8-inch duct handle?
An 8-inch round metal duct comfortably delivers 200 to 260 CFM at standard residential friction rates (0.08 to 0.10 in. wg).