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Measure Your Old Door

In most cases, simply order a new door the same size as the old one. If you alter the size or add sidelights, you'll have to reframe the opening and alter many details. This usually doubles or triples the size of the job. Here are the four sets of measurements you'll need to order a door :

Door size

  • Measure the width and height of your old door.

  • Round these up to full inches to find the size of the replacement door you'll need.

  • For example, if your door measures 35-3/4 in. wide and 79-1/2 in. tall, you'll order a 36-in. by 80-in. door.

Jamb width

  • Measure from the backside of the interior trim to the backside of the exterior trim.

  • Specify this jamb width when you order your new prehung door.

  • This guarantees that the interior trim will fit flush to the wall without adding "jamb extensions."

Rough opening

  • You'll need to remove the interior trim for accurate measurements of the rough opening.

  • Measure the opening width between framing members and from the bottom of the sill to the top of the opening.

  • Compare these measurements to the rough opening requirements of your new door to make sure it will fit.

Exterior opening (or “masonry opening” if you have a brick or stone door surround)

  • Measure to the outsides of the exterior casing, then from the bottom of the sill to the top of the trim.

  • Compare these measurements with those of a prehung door that has standard two-inch-wide “brick molding” trim.

  • If the framed door with standard trim is too small to fill the space, or if you want a different trim style, you have three options.

  • The best solution is a door with wider, flat casing to fit the opening. You can always add a piece of decorative molding overtop to approximate the style of your existing exterior trim. (We ordered 2-1/4-in. flat casing and reinstalled the existing decorative molding.)

  • Second, you can order your door with standard molding and fill the gap with additional strips of wood.

  • The last option is to order the door without exterior molding and make your own to fit.

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