Thread: would a 8" round flex return air duct be enough for a room that has 2 6"...

...and 1 4" forced air vents? my house has one 20 x 30 inch return air grille in the living room. When i shut the door to my master bedroom it slams shut. I want to add a return air in my master to supply air and keep the door from slamming. what size 8" or 10" round flex will work for the return air?

Cut a little off the bottom of the door air can get buy. Won't have the suction to slam the door.