How to Measure for an External Back Door | Framing Your New Door Right

Measure your existing door slab and rough opening at multiple points, recording the smallest dimensions before ordering a replacement.

Measuring for an external back door comes down to a handful of precise numbers. Get them right and your new prehung unit drops into place; get them wrong and you’re staring at a costly return. The process takes about fifteen minutes, a tape measure, and a notepad. The key is knowing which two measurements matter—the door slab and the rough opening—and always using the smallest figure you record. If you’re comparing models and finishes, our tested roundup of the best external back doors pairs well with the measurements you’re about to take.

Why Two Sets of Measurements?

A slab door is just the door itself; a prehung unit includes the door attached to its frame, ready to install into the rough opening. Andersen’s and Masonite’s guides agree on the core principle: slab buyers measure the slab, while anyone installing a whole unit measures the rough opening. Mixing the two is the most common mistake. Grab a tape measure, a pencil, and paper. Masonite also recommends safety glasses and gloves before pulling off interior casing to expose the framing.

Measuring the Door Slab

For swapping just the door into an existing frame, Andersen’s method for width: take three measurements across the door—top, middle, and bottom—and record the smallest as your width. Then measure the height at the left side, center, and right side, noting the smallest of the three. Doors settle and frames shift; no opening is perfectly square, so the smallest reliable dimension accounts for normal wear. One detail often missed: Simpson Door’s guide notes that exterior door measurements get affected by existing flooring. Measure to the bottom of the threshold itself, not the floor surface covering it, since your new door must clear whatever sits below.

Measuring for a Prehung Unit

For a full unit and frame replacement, the rough opening is the number you need. Masonite’s procedure calls for removing the interior casing first to see the bare stud framing on both sides and above the door. For rough opening width, measure between the studs at two or three points—top, middle, bottom—and use the smallest figure. For height, measure from the subfloor (the structural floor beneath finished flooring) straight up to the top of the rough opening, on both left and right sides, and record the smaller measurement. Do not assume the opening is perfectly square; Masonite calls out checking that corners sit at 90 degrees and the floor is level before ordering, since out-of-square openings create fit problems no manufacturer can fix.

Getting the Depth Right

You also need the jamb depth so the new door matches your wall’s thickness. This runs from the interior edge of the frame to the exterior edge, excluding any trim, brickmold, or casing sticking out past the framing. Masonite’s method: measure the side jamb’s depth from its exterior edge to its interior edge; include exterior trim and your new door will sit too deep or too shallow. If your back door has sidelights, measure that surrounding unit separately—sidelights change the rough opening dimensions, and door measurements alone won’t capture the opening’s needs.

What to Do with the Numbers

Once you’ve recorded slab or rough opening dimensions, verify them against your current door, then order your replacement. For standard sized prehung units, comparing steel, fiberglass, and wood options side by side helps narrow the field before you commit to a style and price range. Here’s a quick summary of key figures:

Door sizing standards vary by region, so double-check with your manufacturer or seller on whether units ship in imperial or metric dimensions before finalizing. Your measurements are reliable only if the unit you buy matches the system you measured in.