How to Measure for an Iron Door
Measure the rough opening, not the old door. The rough opening is the framed hole in the wall, and building to it leaves the shim space a pre-hung jamb needs to sit plumb and level.
Take the width at three heights, near the top, the middle and the bottom, and record the smallest. Take the height on both the left and right sides and again record the smallest. Old homes are rarely square, and building to the smallest usable dimension avoids a door that binds.
Measure the jamb depth, the thickness of the wall from interior finish to exterior finish, since the jamb is built to wrap it. If the opening is arched, capture the width, the height to the spring line where the curve begins, and the radius of the arch.
Note the swing you want, inswing or outswing, and the handing, left or right as you face the door from outside. When in doubt, write not sure and we confirm it with you. Download the measuring worksheet to record every reading in one place.