Standard Door Sizes, Rough Openings and the Clear Width You Actually Get

Three numbers, all called "the door size"

Ask what size a door is and you will be told 30 by 80, or 2-6, and that is a real answer to a real question. It is just not the only measurement in play, and the other two are the ones that stop a job.

• The slab is what you buy. 30 inches wide, 80 tall.
• The rough opening is the hole you frame. For that door it is 32 by 82-1/2.
• The clear width is the gap you can actually carry a sofa through. For that door it is about 28-3/8 inches.

Every chart on the internet gives you the first number. The tables above give you all three, because the second one decides whether the door fits the wall and the third one decides whether it passes inspection.

The number the code cares about is the one nobody publishes

The International Residential Code has almost nothing to say about door sizes. It has one requirement, in section R311.2, and it applies to a single door in the house — the egress door:

Clear width of not less than 32 inches, measured between the face of the door and the stop, with the door open 90 degrees. Clear height of not less than 78 inches, from the top of the threshold to the bottom of the stop.

Read that measurement description again, because it is where people come unstuck. It is not the slab. It is not the gap between the jambs. It is what is left after the door has swung into the opening, measured from the face of the open leaf across to the stop on the other side.

An exterior slab is 1-3/4 inches thick. Swing it 90 degrees and it stands in the doorway, taking its full thickness out of the width. The stop on the latch side takes another half inch. So:

clear width ≈ slab width + 1/4 in reveal − slab thickness − 1/2 in stop

Which means a 32 inch front door does not comply

Run the numbers on a 32 inch exterior door and you get 30 inches of clear width against a 32 inch requirement. It fails, and it fails by a margin no inspector is going to wave through.

• 32 in slab → about 30 in clear → does not meet R311.2
• 34 in slab → about 32 in clear → meets it exactly, with nothing to spare
• 36 in slab → about 34 in clear → meets it comfortably

This is the entire reason American front doors are 36 inches. Not fashion, not proportion — it is the smallest stock size that clears the code with margin, and builders stopped thinking about it decades ago. The 32 inch door is fine as a back door, a side door or a garage-to-house door, because those are not the egress door.

The same arithmetic decides accessibility. The ADA standards ask for 32 inches of clear width on an accessible route, and while the ADA does not govern a private house, it is the number everyone sensibly uses when planning for a wheelchair or a walker. A 32 inch interior door gives about 30-3/8 clear and will not do it. You need 34 at the limit, or 36 to be comfortable. If the wall cannot grow, swing-clear hinges move the leaf fully out of the opening and give you back most of what the slab was stealing.

Interior doors have no required size at all

The same code section ends with a sentence worth quoting, because it settles a lot of arguments:

Other doors shall not be required to comply with these minimum dimensions.

That is the whole of the IRC's interest in your interior doors. There is no minimum width for a bedroom door, a bathroom door or a closet. Which is why 24 inch bathroom doors exist in perfectly legal houses, and why "standard" interior sizes are a market convention rather than a rule.

The convention is worth knowing anyway, because it is what stock is made in: 24, 28, 30 and 32 inches do most of the work, 18 and 20 for closets and pantries, 36 where something wide has to move through. Height is 80 inches almost everywhere, with 84 and 96 available and increasingly used in new build.

Rough openings: add two and two and a half

Prehung units want the framed opening 2 inches wider and 2-1/2 inches taller than the slab. That allowance is not arbitrary and it is not padding: it covers the jamb stock on both sides and the shim space you need to plumb and square the unit in a wall that will not be either.

So a 30 by 80 prehung wants 32 by 82-1/2, and a 36 by 80 wants 38 by 82-1/2.

Two things that go wrong here regularly:

• Measuring an opening that already has a door in it. The rough opening is behind the jambs and casing. Measure stud to stud and header to subfloor, not jamb to jamb.
• Forgetting the finished floor. The height allowance assumes the door sits on the finished surface. If tile or 3/4 inch hardwood is going in after the door, that height comes out of your undercut, and you will be trimming the bottom of a new door on your knees.

The shorthand, in case nobody has explained it

Door sizes get written as 2-8 or 3-0. It is feet and inches with the foot mark dropped, so 2-8 is 2 feet 8 inches — 32 inches — and 3-0 is 36. A schedule reading 2-6 x 6-8 is asking for a 30 by 80.

It is worth being fluent both ways. The shorthand is what a supplier will quote you and what a plan will call out; inches are what you will actually hold a tape to. Getting the two confused is how a 2-4 gets ordered for an opening that wanted a 3-0.

Before you buy anything, measure the opening you have

Charts are for planning. The door goes in a specific hole in a specific wall, and that wall was framed by someone having a specific kind of day.

Measure the rough opening at the top, the middle and the bottom, and use the smallest figure. Check the diagonals to see how far out of square it is. Then pick the door that fits what you measured, because re-framing an opening is a day of work and buying the next size down is five minutes.

And if the door in question is the one you leave the house by, put a tape across the finished opening with the door standing open at 90 degrees. That single measurement is the one the code is actually asking about, and it is the only one that settles it.