How to Measure When Replacing Doors

Create a sketch of your door. Use this to record measurements to make it easier.

Remove all interior trim, using a hammer or pry bar. Take care not to damage the trim so you can reuse it later.

Measure the door size -- both the height, from the bottom of the threshold to the top of the door, and the width of the old door. Round the measurement up to the next full inch; a 36 3/4-inch wide by 83 1/2-inch high door would thus be considered a 37-inch by 80-inch door. For interior doors, this is the only measurement you will need, other than thickness -- most interior doors are 1 3/8-inch thick, but double check on older or custom-made doors.

Determine the jamb width -- the wood frame surrounding the door, to which the door hinges attach. Measure from the interior side to the exterior side of the jamb -- the depth of the wood from inside to out in other words. Proper measurement of the jamb width ensures your interior trim will install flush against the wall when a new door is installed.

Size the rough opening, between the door framing -- typically the 2-by-4 studs on either side of the door, sometimes brick or other materials -- on either side of the door width. Also measure from the bottom of the sill, which is the lowest part of the door assembly, to the top of the opening, but not the door itself.

Measure the exterior opening, from the outside of the exterior casing, which is the part of the door that attaches to the wall, to the opposite, as well as from under the sill to the top of the trim above the door. Casing size will prove a factor when replacing some doors, unless you modify trim in the new installation.

Note the door thickness for the final measurement. Most exterior doors are standardized at 1 3/4-inch thick, but older and custom doors may vary.