The size of the duct needed for a commercial kitchen exhaust depends on the CFM rating of the range hood: 8-Inch Duct: Suitable for range hoods with CFM ratings between 400 and 900 CFM, commonly used in medium-sized kitchens or with moderately powerful range hoods. 10-Inch Duct: Suitable for range hoods with CFM ratings above 900 CFM, typically used in larger kitchens or with high-powered commercial-grade range hoods.