How do I measure for a new front door?

Should I replace just the door or the whole unit?

Replace just the door, a slab, when the existing frame is sound, square and the size you want to keep. Replace the whole prehung unit, the door already hung in a new frame (the jamb) with hinges, a sill and weatherstrip, when the frame is worn, the sill leaks or you want a different size.

Simpson Door Company suggests replacing the entire door system, including the jamb and sill, when you want a new look, wood or style, or when the jamb or frame is worn or damaged (Simpson). Older houses often have doors that are an odd size or thickness; then the new door has to be made to fit the frame, or the frame replaced.

What size is a standard front door?

36 inches wide by 80 inches tall, and 1-3/4 inches thick. Pella says standard front doors typically measure 36 by 80 inches, with entry doors ranging from 30 to 45 inches wide and 71 to 96 inches tall (Pella).

Size changes the hardware too. Simpson specifies three hinges on doors up to 7 feet tall and four on taller doors (Simpson). If you choose a door thicker than 1-3/4 inches, check that the lockset suits it.

How small can a front door be?

For the main exit door of a house, not smaller than 32 inches of clear width and 78 inches of clear height. The 2024 International Residential Code, section R318.2, requires at least one side-hinged egress door per dwelling unit with a clear width of not less than 32 inches "measured between the face of the door and the stop, with the door open 90 degrees", and a clear height of not less than 78 inches "measured from the top of the threshold to the bottom of the stop" (2024 IRC R318.2).

Those are clear-opening sizes, not door sizes: a 36-inch door leaves less than 36 inches clear once it is open. Longmont adopted the 2024 International Codes as of July 1, 2026 (City of Longmont), and the City of Boulder adopted them effective Aug. 1, 2025 (City of Boulder). We found no local change to this section in either city's code, but your building department has the final word.

How do I measure an existing door?

If you are keeping the frame, measure the door itself. Simpson's method (Simpson):

1. Measure the height at the left edge, the center and the right edge, leaving out the door sweep. Write down the tallest number.
2. Measure the width near the top, middle and bottom. Write down the widest number. Most exterior doors measure 36 inches or close to it.
3. Open the door and measure its thickness at the edge. Exterior doors are typically 1-3/4 inches.
4. Work out the handing and swing (below).

Then add what a maker needs to machine a new door to your frame: where the hinges sit (top of each hinge from the top of the door), where the lock and deadbolt holes are, and the backset, the distance from the door edge to the center of the lock hole.

A slab can only be trimmed a little. Simpson, for example, says you can cut a maximum of 1/2 inch from the top and 1-1/2 inches from the bottom of its doors (Simpson FAQ). Order the size you measured rather than planning to cut a door down.

How do I measure the rough opening and jamb depth?

For a full replacement, take the interior trim off so you can see the framing. Masonite's method (Masonite):

1. Measure the width of the rough opening from stud to stud at more than one height. Write down the smallest number.
2. Measure the height from the subfloor to the top of the rough opening on each side. Write down the smaller number.
3. Measure the jamb depth from the exterior edge of the side jamb to the interior edge, leaving out the brickmould and casing.

Rough openings are larger than the door to leave room for the frame, shims and sill, and the allowance varies by maker. Sun Mountain subtracts 2 inches from the rough opening width and 3 inches from the height, so a 38-inch-wide opening takes a 36-inch door (Sun Mountain). Masonite lists 38-1/2 and 34-1/2 inches as common rough opening widths and 82-1/2 and 98-1/2 inches as common heights (Masonite). Use the rough opening on the spec sheet for the unit you order.

Other makers list slightly different stock depths: Masonite gives 4-9/16, 5-1/4 and 6-9/16 inches as common side jamb measurements (Masonite), and Simpson says most come close to 4-9/16 or 4-5/8 inches (Simpson). If your wall measures something else, the jamb can be made to suit; Sun Mountain, for one, makes custom jamb sizes on request (Sun Mountain).

Is my door left-hand or right-hand?

Stand in the doorway with your back against the hinge side. If the door opens to your left, it is a left-hand door; to your right, a right-hand door. If it swings into the house it is an inswing door, and if it swings out it is an outswing door (Sun Mountain).

Naming conventions vary within the industry. Simpson recommends explaining how you worked out the swing and sending photos of the door open in its frame (Simpson).

What should I measure besides the door?

Two things that decide which wood and finish will last: which way the door faces, and how much roof is over it. A door maker in Colorado should ask about both.

• Which way the door faces. Stand outside facing the door and note the compass direction you are facing; the door faces the opposite way. A phone compass is fine. South and west faces get the most sun.
• Overhang depth and height. Measure how far the porch roof or overhang projects out from the wall, and how high its underside is above the bottom of the door. Simpson calls an overhang adequate when the projection is at least half that height (Simpson): a roof 8 feet above the sill should project at least 4 feet.
• The floor at the threshold. Simpson measures an exterior unit's height from the bottom of the threshold, so new flooring against it has to be allowed for.

Which photos should I send?

1. The entry from the street, straight on, so the door can be drawn in proportion to the house.
2. The door from inside and outside, open and closed, showing the hinges and the lock.
3. The overhang or porch roof above the door, from the side if you can.

Write each number down with where you took it, and keep door measurements and rough opening measurements separate. With those and the photos, a door maker can draw and price your door. Get a door quote.