How to measure a door

Door sizes get confusing because people measure different things. Here's what each word means:

• Slab: the door itself. A "36-inch" exterior door actually measures 35-3/4" wide and 79" tall.
• Frame (the unit): the jambs, head and threshold the door hangs in. A prehung door, or door system, comes with the door already hung in its frame.
• Rough opening: the framed hole in the wall, stud to stud and floor to header. It's a little bigger than the frame, so there's room to shim it plumb and square.
• Jamb depth: how far the frame reaches through the wall. It should match your wall's thickness.
• Handing: which side the hinges are on, and which way the door swings.

1. Score the paint along the inside trim (the casing) with a utility knife and pry it off. Go slowly and you can reuse it.
2. Width: measure stud to stud at the top, middle and bottom. Where the old frame is in the way, measure the frame's outside width and add the gap on each side. Use the smallest number.
3. Height: measure from the subfloor, where the threshold sits, to the underside of the header, on the left and the right. Not from the top of the old threshold: it sits on the subfloor and is usually 1" to 2" tall. If it is in the way, measure to its top and add its height. Use the smaller number.
4. Square: measure both diagonals, corner to corner. Within 1/8" is square; up to 1/4", an installer can shim it out.
5. Wall thickness: measure from the inside wall surface to the outside of the sheathing.

Your new frame should be 1/2" to 1-1/2" narrower and 1/4" to 1" shorter than the rough opening. The tool does this math for you.

You can get a good estimate without taking anything apart:

1. Open the door and measure the door itself: edge to edge, and top to bottom.
2. Measure between the jambs, where the door closes, at the top, middle and bottom.
3. Measure the jamb front to back, not counting the trim.
4. From outside, measure across the brickmold, outside edge to outside edge.

A new door system the same size as your old one fits the same opening in almost every home. Confirm the rough opening before you order, or send us your measurements and photos and we'll review them for free. Guaranteed Fit covers door systems measured at the rough opening.

A new door has to match your frame exactly, so measure the old door and the frame:

1. The old door: width, height and thickness. Most exterior doors are 1-3/4" thick, and most interior doors 1-3/8".
2. The frame: between the jambs at the top, middle and bottom, and from the head jamb to the threshold on both sides.
3. Hinges: count them, measure one hinge's height (3-1/2", 4" or 4-1/2"), and measure from the top of the door to the top of each hinge. Check the corners: square, or round like a dime (1/4") or a quarter (5/8").
4. Lock: count the holes, measure the backset (the door edge to the center of the hole, usually 2-3/8" or 2-3/4"), and measure from the top of the door to the center of each hole.

We prep the new door to match. If your frame is rotted or out of square, or the door never latched well, replace the whole door instead: a new slab can't fix the frame.

If the wall is framed, measure the rough opening the same way as for a replacement. If it isn't framed yet, pick your door first and frame to its rough opening. The tool gives you the exact size, and the table below lists the common ones.

Frame the header level and the studs plumb, and make sure the floor where the threshold sits is flat and level.

Stand inside your home, facing the door:

• Inswing doors (opening toward you) are named by the handle side. Handle on the left: a left-hand inswing (LHIS).
• Outswing doors (opening away from you) are named by the hinge side. Hinges on the left: a left-hand outswing (LHOS).

Interior doors are named from the side the door swings away from you: hinges on the left is a left-hand door.

You don't need to remember any of this. The tool asks two simple questions and works it out. Handing can't be changed once a door is built, so it's worth a second look.

The jamb should be as deep as your wall, so the trim sits flat on both sides. Replacing a door? Just lay the tape across your old jamb, front to back: it was made to match the wall. Measure from the inside wall surface (drywall or plaster) to the outside of the sheathing, not the siding or brick.

• 2x4 wall with 1/2" drywall and 1/2" sheathing: about 4-1/2". Use a 4-9/16" jamb.
• 2x6 wall: about 6-1/2". Use a 6-9/16" jamb.
• Stucco: about 5-1/4" on a 2x4 wall and 7-1/4" on a 2x6 wall.
• Anything in between (plaster, thicker sheathing, foam board): we cut jambs to any depth from 3-9/16" to 8-9/16".
• Deeper walls, such as brick or block: add a jamb extension on the inside.