Standard Door Size | Standard Door Height and Width

Four measurements that are easy to confuse

Slab size is the door panel’s width, height and thickness. Unit size is the outside dimension of a complete frame assembly. Rough opening is the opening in the framing that receives that assembly. Clear opening is the usable passage with the door open.

These are not interchangeable. A nominally wide door can provide a narrower clear passage because the open slab, stop and hardware occupy space. A rough opening also needs the manufacturer’s prescribed installation allowance around the frame.

Replacing only the slab

Measure the existing slab in several places and record the smallest and largest readings if they differ. Then record thickness, hinge size and positions, hinge corner shape, lock-bore locations, backset and swing. Check the jamb for damage or distortion before deciding to reuse it.

A slab with the correct overall dimensions may still need machining to match the existing frame. Do not trim a door until you know its construction and the permitted trimming allowance. Glass, hollow-core construction, metal skins and rated assemblies each impose different limits.

Replacing a prehung assembly

Use the selected product’s unit dimensions and required rough opening. Measure the framing opening at the top, middle and bottom, then measure height at both sides and the center. Check diagonals, plumb and level; a wide measurement at one point does not make an out-of-square opening ready for installation.

Agree on the floor reference with your installer. Finish flooring, underlayment, threshold height and the sill detail can change the available height. Keep a photograph or sketch showing where every measurement starts and ends.

What does jamb depth mean?

Jamb depth is the distance through the wall that the jamb needs to cover. It is not the width between the side jambs. Include the wall’s finished surfaces as required by the frame design, and distinguish decorative casing from the jamb itself.

Record the actual finished-wall depth rather than choosing a jamb from the framing lumber name alone. The door jamb guide explains the frame parts and their relationship to trim.

Accessibility and building requirements

Stock dimensions are product conventions, not universal code requirements. Egress, accessibility, fire protection and other rules depend on the opening and the applicable standards. Have the project designer or installer confirm them before changing an opening.

For accessible routes, check clear passage, maneuvering space, threshold, hardware and operating conditions together. Ordering a door advertised as a particular width is not by itself evidence that the finished opening complies.

Barn, pocket and folding doors need separate sizing

A barn door typically covers an opening from the wall face; a pocket door moves into a cavity; folding panels stack at the opening. Their track, overlap and clearances differ from a hinged prehung door. Use the guide for the actual system rather than transferring dimensions from another door type.

Questions worth asking before you choose

Can I add two inches to a door size to get the rough opening?

Do not treat that as a universal rule. Jamb construction, sill details and installation allowances differ. Use the manufacturer’s rough-opening dimensions for the exact unit.

What if my opening is not a stock size?

First establish whether the frame can be reused or needs replacement. Then compare a suitable custom unit with altering the opening, including structural work and finish repairs. A custom slab still needs compatible hinge and lock preparation.