ducting calculation

I am replacing an old noisy bathroom exhaust fan that currently has a 4" roof jack/cap and 4" ductwork. According to the proper calculations, I would need a new exhaust fan rated at 190 CFM. However, I can't find any 4" exhaust fans with CFM ratings this high. The roof is 7/12 pitch and I...

I have a question about how to calculate duct size when there are multiple supply or return vents in the same room. Say I have a room that requires 400 cfm. Using a ductulator for flex ducting tells me I need 11" diameter. Now if I have two vents at opposite sides of this room, how do I...