ACCA Manual D • 0.1 in. wg Friction600 CFM • 1.5 TonsWhisper Quiet (<700 FPM)
600 CFM Duct Size: 1.5 Ton HVAC Trunk Line
Calculated duct sizing specifications for 600 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 Velocity764 FPMFeet Per Minute
Cooling Tonnage1.5 Tons@ 400 CFM / Ton
Register / Grille12x18Face 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 |
|---|---|---|---|---|---|
| 8" Deep | 18" Wide | 144 sq in | 600 FPM | 2.3:1 | Optimal (<2:1) |
| 10" Deep | 14" Wide | 140 sq in | 617 FPM | 1.4:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:12x18 Trunk Plaque / 14x20 Return Grille
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 round duct delivers 600 CFM at 764 FPM velocity, well under residential velocity limits (<900 FPM).
- For rectangular ducting in a basement ceiling, an 8x18 or 10x14 duct fits neatly between standard floor joists.
Frequently Asked Duct Sizing Questions
What size duct is needed for 600 CFM?
A 12-inch round duct or an 8x18 rectangular trunk is standard for 600 CFM (1.5 tons of air conditioning).