Duct Sizing Chart CFM

Quick answer: duct size depends on CFM, duct type, friction rate, equivalent length, fittings, static pressure, and acceptable noise. A quick field chart may put 8-inch round metal duct around 180 CFM, 10-inch around 300 CFM, and 12-inch around 480 CFM, but final HVAC design should use a Manual D / ductulator calculation.

Round Duct Size Chart

Round duct size	Approx. metal duct CFM	Approx. flex duct CFM	Typical note
5 in	50 CFM	50 CFM	Small branch run
6 in	85 CFM	75 CFM	Small room branch
7 in	125 CFM	110 CFM	Medium branch
8 in	180 CFM	160 CFM	Common branch/trunk transition
9 in	240 CFM	225 CFM	Large branch
10 in	325 CFM	300 CFM	Small trunk
12 in	525 CFM	480 CFM	Trunk line
14 in	750 CFM	700 CFM	Trunk line
16 in	1,200 CFM	1,000 CFM	Main trunk
18 in	1,500 CFM	1,300 CFM	Main trunk
20 in	2,000 CFM	1,700 CFM	Main trunk

Rectangular Duct Size Examples

Design airflow	Example rectangular sizes	Approx. round equivalent	Note
100-150 CFM	8 x 6 in, 10 x 4 in	6-7 in round	Small branch
200-300 CFM	10 x 8 in, 12 x 6 in	8-10 in round	Medium branch
400-500 CFM	14 x 8 in, 16 x 8 in	11-12 in round	Large branch/small trunk
700-800 CFM	18 x 10 in, 20 x 8 in	14 in round	Trunk duct
1,000-1,200 CFM	20 x 12 in, 24 x 10 in	16 in round	Main trunk

Before Using A Duct Size Chart

Account for elbows, takeoffs, registers, filters, dampers, and flex duct compression.
Keep air velocity reasonable to avoid noise.
Use a duct calculator or Manual D process for final design.