This is a 4" Inside Diameter Unit - 1.00" to 1.50" Thickness Range. You will need to have a hole that ranges from 4 7/8" - 5 1/8" for installation, with 5" being our ideal installation hole size. This is a 2-piece unit that screws together from each end of the barrier you are penetrating, so you much have access to both sides of the object (such as a wall, floor, desk, etc.).