HVAC Duct Sizing & Airflow CFM Matrix
Eliminate noisy air rushes, weak room airflow, and frozen evaporator coils. Select your desired CFM airflow or tonnage below to get exact round pipe diameters, equivalent rectangular dimensions, maximum velocity ratings, and filter grille sizes.
Main Supply Trunks & Tonnage Plenums (400 to 2,000 CFM)
Central furnace and air handler trunks for 1.0 to 5.0 ton split systems (0.10 in. wg friction rate)
1.0 Ton Cooling Zone Sub-Trunk
1.5 Ton Cooling Sub-Trunk Line
2.0 Ton Main Supply Trunk
2.5 Ton Main Supply Trunk Line
3.0 Ton Whole-House Main Supply Trunk
3.5 Ton Whole-House Supply Trunk
4.0 Ton Main Supply Plenum & Trunk
4.5 Ton Main Supply Trunk Line
5.0 Ton Maximum Residential Supply Trunk
Return Air Drops & Filter Grille Sizing (Low Velocity <600 FPM)
Engineered return air paths to prevent blower starvation, whistling grilles, and frozen coils
Individual Room Branch Runs (50 to 350 CFM)
Branch takeoffs for bedrooms, bathrooms, kitchens, and living rooms (0.08 in. wg friction rate)
Bathroom Exhaust / Powder Room Branch
Bedroom Branch Run (80–120 sq ft)
Master Bedroom Run (150–220 sq ft)
Large Living Space Branch (250–300 sq ft)
Kitchen / Great Room Branch (300–350 sq ft)
Great Room High-Demand Branch
Upper Floor Bonus Room (400–500 sq ft)
Flexible Duct CFM Capacity Guides (6" to 14")
Practical airflow limits for flexible duct accounting for core compression and sag
6-Inch Flex Duct CFM Capacity & Sizing Guide
8-Inch Flex Duct CFM Capacity & Sizing Guide
10-Inch Flex Duct CFM Capacity & Sizing Guide
12-Inch Flex Duct CFM Capacity & Sizing Guide
14-Inch Flex Duct CFM Capacity & Sizing Guide
All duct sizing calculations utilize the standard ASHRAE / Equal Friction Formula: D = ((0.10913 × CFM^1.9) / FrictionRate)^(1/5.02) with standard friction rates of 0.08 to 0.10 in. wg per 100 equivalent feet. Equivalent rectangular sizes are solved via the Huebscher Equation: De = 1.30 × (a × b)^0.625 / (a + b)^0.25 maintaining an aspect ratio of less than 4:1. All installations must comply with the International Mechanical Code (IMC) and local building authority jurisdictions.