Can a single 8" flex duct be split into a 5" flex and 7" flex?
The volume of a round duct can be generally estimated using its cross section. Pi * r^2
For an 8&quot; it's 16Pi
7&quot; is 12.25Pi
5&quot; is 6.25 Pi.
18.5 &gt; 16, so this is an oversized split.
However, using a 6&quot; 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&quot; would be 25Pi, and that will happy split into 7+7 or 8+6.