This Liberty Hardware allows you to turn light weight interior doors into self-closing doors. No need to remove door for installation. Long lasting construction features adjustable spring and nylon bearing to reduce friction. A 30 pound door such as a wood hollow core door will close with one unit according to the manufacturer, however, we recommend using one unit per hinge. In our office, I installed these on a 36 inch commercial steel door which has three hinges. I installed one for each hinge. This works great. All of these closers from various manufacturers come from the same factory in China-- Ace Hardware, Stanley, or whatever. I mention this because I discovered something while installing mine, and I do not want you to think that it is a something unique to our unit-- you must have 1/2" between the edge of the hinge leaf and the door facing to a;;ow room for the "clip" that snaps over the hinge leaves. See Pictures and read "Technical Info".