How to Calculate the Size of an Interior Door

Score the seam where the door trim meets the wall with a utility knife. This cuts through paint and caulking that might interfere with prying the trim away from the doorway.

Slide a small pry bar between the wall and trim. Gently tap the pry bar with a hammer to begin prying the trim off the wall. Lift the trim a little at a time and move up the length of the trim. Once you are able to see the nails or staples securing the trim, pry where the nails are and remove the trim completely from the doorway. Repeat the process for the remaining trim.

Measure between the studs on each side of the opening with a tape measure. The studs are visible once you remove the trim. Measure at the top of the doorway and the bottom of the doorway. Use the smallest measurement as the rough opening width.

Place the end of the tape measure under the header board and measure down to the floor. Take this measurement from the left and right sides of the doorway. The smallest of the two measurements is your rough opening height.

Open the door and measure the width of the existing frame with the tape measure. This gives you the thickness of your wall. Take these measurements with you when purchasing replacement interior doors.