nealtw
Contractor retired
Are you calculating area in the cabinet or cubic in. in the pipe?
Your installation could have as much to do with the 90s as with the pipe diameter, and you didn't tell us the size of the unit, so YMMV.
What do want to know about the size of my unit?
Size does matter and yes mine would calculate as 13 ft with the 90*s
32.5 sq. inches is what you have with the micro wave over the stove so changing to 4" cuts the sq. inches to almost a third. Anytime it is straight forward and easy, go with 6"