How to Measure a Door
Measure the width at the top, middle, and bottom of the opening and use the narrowest number; measure the height at both sides and the middle and use the shortest. Also measure the door slab's thickness and the wall's depth.
Measure the door opening’s width at the top, middle, and bottom, and use the narrowest of the three — that’s the tightest point a new door has to fit through. Do the same for height at both sides and the middle, using the shortest number. Then measure the door slab’s thickness (or the wall’s depth, if you’re installing a full pre-hung unit) depending on what type of replacement you’re buying.
Slab replacement vs. pre-hung replacement
- Slab door: just the door panel, no frame. Used when your existing frame and hinges are in good condition and you’re only replacing the door itself. Measured against the existing frame’s opening dimensions.
- Pre-hung door: the door, frame, and hinges as one assembled unit, sometimes with casing included. Used when the frame itself needs replacing too. Measured against the rough opening in the wall structure, which is typically about 2 inches wider and 2-2.5 inches taller than the door slab’s own size, to leave room for the frame and shimming.
Knowing which one you need before measuring matters, since the two measurement targets (existing frame vs. rough wall opening) are different.
Step-by-step for a slab replacement
- Width: measure the frame opening’s width at the top, middle, and bottom. Use the smallest number.
- Height: measure at the left side, right side, and middle. Use the smallest number.
- Thickness: measure the existing door slab’s thickness — 1 3/8” is standard for interior doors, 1 3/4” for exterior.
- Hinge and hardware positions: note the existing hinge count, size, and placement, along with the door handle’s height and bore-hole diameter, so the new slab’s pre-cut hardware (if any) lines up.
Standard sizes as a reference point
| Door type | Standard width | Standard height | Standard thickness |
|---|---|---|---|
| Interior (bedroom/bath) | 28-32 in | 80 in | 1 3/8 in |
| Interior (primary/hallway) | 30-36 in | 80 in | 1 3/8 in |
| Exterior | 36 in | 80 in | 1 3/4 in |
These are common defaults, not universal — always measure your specific opening and existing door rather than ordering to a standard size, especially in older homes where doors were sometimes built to non-standard dimensions.
A quick squareness check
As with windows, it’s worth measuring both diagonals of the opening corner to corner — if they’re within about 1/4 inch of each other, the frame is close enough to square for a standard installation. A bigger gap is worth flagging to whoever’s doing the installation, since it may need extra shimming or adjustment.