Jerome is right at 14 1/4" each way, but if it is a power vent, you need to leave more room in one direction for your wiring (about an inch will do). Also, if you are using 1" framing, the pre-drilled holes in the flanges of the vent may extend out beyond your framing. You will have to either drill new holes or add some scab pieces to catch the existing holes. Best to lay it all out ahead of time before you finalize your framing. Larry S.