I ordered 6" to make a 6" semi-rigid flexible duct to connect the built in duct to the fan. Too small, literally 6 inches as noted by another reviewer (review came after I ordered mine). Had I known that I would have ordered the size 8" instead. It would be fine, probably for a 5" diameter duct or less. I don't know if I'll be returning them or just for some unknown future use For my current needs, I had to make a trip to a big store and buy 5-7" clamps, which worked fine.
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