Flexible Duct Cfm Chart A rectangular 6×6 inch duct has an airflow of 110 cfm. bigger 6×12 inch ducts can handle 270 cfm airflow. the bigger the crosssection of ducts, the bigger cfm; that’s pretty obvious. the smallest 6×4 ducts can handle 60 cfm airflow. the biggest 42×12 ducts with 504 sq inch crosssection can handle 3000 cfm airflow. Using the chart, you’ll see that for a 12” round flex duct, maximum cfm is 480 cfm or 500 cfm depending on the friction rate. duct size for 115 cfm. find the cfm closest to 115 in the table above. it will be found in different columns depending on the friction rate. if there isn’t an exact match, go up to the duct size.
Cfm Flex Duct Chart 22031 2 x30235040x10290040x12. 305042x12. flex duct = .05" on most metal duct calculator round metal pipe = .06" on most metal duct calculators rectangular sheet metal duct = .07" on most metal duct calculators. duct size. duct height net inside dimension in inches. We have included charts for most of the popular standard air duct sizes. these include: round flex duct: 5 inch – 20 inch. metal round pipe: 5 inch – 20 inch. 4 inch rectangular duct: 6 inch – 24 inch diameter. 6 inch rectangular duct: 4 inch – 30 inch diameter. 8 inch rectangular duct: 4 inch – 36 inch diameter. Now, multiply 3 x 400 cfm to get 1200. you would need a blower that can move 1200 cfm. pro tip: round up – if you’re more than halfway to the next larger size, round up to it. in our example, if you needed 33,500 btu, or 2.75 tons, round up to 3 tons and multiply by 400 cfm per ton for the same result – 1,200 cfm. It means that for every 100 ft, the friction loss is 0.3 in.wg. if your duct length is not 100 ft, the friction loss is reduced proportionally. for instance, if the longest duct (from the blower to the farthest vent) is 30 ft, a 6″ duct carrying 200 cfm will result in a friction loss of 30 ÷ 100 x 0.3 = 0.09 in.wg.
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