Our house used to have a fur-down A/C unit that fed a series of rigid ducts made with duct board. We'd like to replace a vent in our bathroom (5' x 11') with a 5" flex duct. We'd like to replace a vent in our kitchen with a 7" flex duct.

Based on the research I've done, It seems we can take a single 8" flex duct off the output plenum and split it into a 5" flex and 7" flex.

Do all those sizes seem appropriate for the rooms? Can I really split an 8" flex into a 5" flex and 7" flex?

I'm also wondering if 7" would be big enough for the kitchen so I'm considering going with a 8" flex. So could I run a 10" flex and split it into a 5" flex and 8" flex?

Are there any issues with using a flex duct as a "trunk" line to the splitter? Or would it be better to use a rigid duct to the splitter and then go with flex lines from there?

1 Answer 1


The volume of a round duct can be generally estimated using its cross section. Pi * r^2 For an 8" it's 16Pi 7" is 12.25Pi 5" is 6.25 Pi.

18.5 > 16, so this is an oversized split.

However, using a 6" instead of a 7 gives you 9Pi. 9+6.25=15.25

So splitting an 8 into a 5+6 is a bit smaller, a 5+7 a bit bigger. Given the choice I'd go 5+7 but if the rooms require 5+7 then it will be insufficient.

A 10" would be 25Pi, and that will happy split into 7+7 or 8+6.

  • This is great information. Thank you. Forgive me for asking, but whether the room requires 5+7 comes down to what? System load balancing and CFMs and ACHs and FPMs and all the other intricacies in balancing a system? Sep 17, 2020 at 17:32

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