The Correct Width for a Doorway Opening

Slab and Prehung Doors

• For the purpose of determining width for installation, there are two types of doors. A slab door has nothing else attached to it and must be installed into a frame in your home. Prehung doors are already installed inside of a frame. These are typically much easier to install because they are already fitted and level in the frame and are ready to put into a doorway opening.

Back-to-Front Width

• Prehung doors have standard frame widths, and the optimal width depends upon the width or thickness of the walls. Standard frame widths, front to back, are 4 9/16 inches and 6 9/16 inches. The former is appropriate for walls with two-by-four studs (the extra width taking drywall into account) and the latter is for two-by-six studs. This is also known as the jamb width. A slab door will need a frame, or jamb; you can use these same guidelines to determine the optimal width for the doorjamb of your slab door.

Side-to-Side Width

• Unless you order a custom door, it will come in a standard width; while there is some variation, most doors are 3 feet wide, or 36 inches. In general, for a prehung door, add 2 to 2 1/2 inches to the width of the door to get the appropriate width for the doorway opening. Doorjambs are typically 3/4 inch thick, so this allows room for installation as well as natural expansion of the wood. The same will apply to a slab door, since the jamb for a slab door will be similar in size. For a standard 36-inch door, then, the appropriate width would be 38 to 38 1/2 inches.

Double Door Widths

• If you have a double door, widths front-to-back are going to be similar to that of a single door. The overall width of the door opening will be the width of both doors together (or twice that of a single door) plus 2 to 2 1/2 inches. Don't add 4 to 5 inches, or double what you would need for a single door; you only need to accommodate doorjamb width and expansion on one side of each door.