Turtle,

The resistance to air flow is higher with flex due to corrugated inside
surface in flex VS smooth surface with sheet metal.

ACCA sells a ductulator (duct calculator circular slide rule with
scales for: Sheet metal, flex, duct board, lined sheet metal). Check
out the same air flow with the same static; the flex will be one size
bigger than sheet metal. The above ductulator is $45.00 to non members
like me.

Just bought one because I lost mine. Bought it at their online store.

Stretch