ACCA Manual D • 0.1 in. wg Friction400 CFM • 1 TonsWhisper Quiet (<700 FPM)
400 CFM Duct Size: 1.0 Ton HVAC Trunk & Zone Supply
Calculated duct sizing specifications for 400 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 Size10" RoundSpiral / Snaplock Pipe
Air Velocity733 FPMFeet Per Minute
Cooling Tonnage1 Tons@ 400 CFM / Ton
Register / Grille10x14Face Dimension
Equivalent Rectangular Duct Sizing Schedule
Exact rectangular sheet metal dimensions offering equivalent airflow resistance to a 10" round pipe
| Height (Depth) | Width | Cross Area | Air Velocity | Aspect Ratio | SMACNA Status |
|---|---|---|---|---|---|
| 8" Deep | 14" Wide | 112 sq in | 514 FPM | 1.8:1 | Optimal (<2:1) |
| 10" Deep | 12" Wide | 120 sq in | 480 FPM | 1.2:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:10x14 Supply Trunk / 12x12 Filter 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
- Industry standard rule of thumb: 1.0 Ton of AC requires 400 CFM of nominal airflow.
- A 10-inch round duct delivers 400 CFM at 733 FPM, serving as the feeder trunk for two to three bedroom branch takeoffs.
Frequently Asked Duct Sizing Questions
What size duct is needed for a 1-ton AC unit?
A 1.0-ton AC (400 CFM) requires a 10-inch round duct or an 8x14 rectangular trunk line.