Determine the size of the door.
Normal, single interior doors are 22" - 42" wide and 6' 8" tall. Take into consideration what types and sizes of items you will be placing in the room. For example if the door leads to a laundry area, make sure the door will be large enough to fit a washer and dryer through, and preferably 36" wide.
Determine the size of the door opening
The size of the door opening will vary depending on the size of the door you plan to install. A typical door opening is 2 inches (5.1 cm) wider than the door size being installed to allow for the thickness of the jamb material and shims to plumb the jamb.
· Measure your door carefully and cut the opening to fit using a reciprocating saw.
· Make the opening the width of the door plus 2 inches (5.1 cm), and if additional studs are needed, a space for these, as well.