ACCA Manual D • 0.08 in. wg Friction100 CFM • 0.25 TonsWhisper Quiet (<700 FPM)
100 CFM Duct Size: Small Bedroom & Home Office Branch
Calculated duct sizing specifications for 100 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 Velocity509 FPMFeet Per Minute
Cooling Tonnage0.25 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 |
|---|---|---|---|---|---|
| 6" Deep | 6" Wide | 36 sq in | 400 FPM | 1.0:1 | Optimal (<2:1) |
| 4" Deep | 10" Wide | 40 sq in | 360 FPM | 2.5:1 | Acceptable (<3:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:4x10 or 4x12 Floor/Ceiling 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
- A 6-inch round duct is the preferred industry standard for 100 CFM to guarantee whisper-quiet operation in bedrooms (<550 FPM).
- If using flexible duct, ensure the inner core is pulled 100% tight; a 15% sag can reduce airflow by 30%.
Frequently Asked Duct Sizing Questions
What size duct do I need for 100 CFM?
A 6-inch round duct or a 4x10 rectangular duct is the recommended standard for 100 CFM, keeping air velocity around 500 FPM.