Even if the old door frame is fine, the wall studs it's nailed to can bow and settle out of square. This makes it difficult to open and close the door. To make a new wood door fit .... ( MORE BELOW )
 

an out-of-kilter frame, you'll need to plane the top and bottom or even trim one of these edges so the door hangs correctly. This is only an option with a wood door; metal and fiberglass doors can't be planed or cut.

Most new doors are pre-hung, which means the door hangs on hinges within a new frame (these systems also include some form of weather-stripping). Pre-hung doors are an ideal choice if the old frame is bad or if you're removing the frame because you want to enlarge the opening.

If you're replacing your old door with a pre-hung unit, first determine if you need a left- or right-hand door. Stand in the doorway and face outside. If the lockset is on your right, you have a right-hand door.

To choose the proper jamb width, measure the existing door jamb between the inside edges of the exterior and interior casing. The width of the door is measured across its face. Most doors are sold as 3-0 (36 in.) or wider.

An alternative to replacing the entire frame is to use door-replacement kits, such as Replace Door Systems from from Pease Industries. Here, the door is prehung in a small steel frame that attaches to the old one. Benefits include easy installation and the added security of the steel frame. However, these kits slightly reduce the original opening, they're available in only a few sizes and they can't be installed over rotted jambs.