Measuring the rough out of an opening is important for selecting the correct frame size and ensuring that the frame will fit properly in the opening. Here are the steps to measure the rough out of your opening:
Remove any trim or casing around the door frame so that you can see the rough framing.
Measure the width of the rough opening. Measure the distance between the inside faces of the framing on either side of the opening. Measure in three places: at the top, in the middle, and at the bottom of the opening. Record the smallest measurement.
Measure the height of the rough opening. Measure the distance between the top of the rough sill (or the subfloor if there is no sill) and the bottom of the header. Again, measure in three places and record the smallest measurement.
Measure the depth of the rough opening. Measure the distance from the inside face of the exterior wall to the inside face of the interior wall. This will give you the thickness of the wall.
Check for squareness. Measure the diagonals of the rough opening. If the two measurements are the same, the opening is square. If not, you will need to make adjustments to the framing before installing the door.
Use these measurements to select a frame that will fit the rough opening. Keep in mind that the frame should be slightly smaller than the rough opening to allow for shimming and leveling during installation.