The jamb width is the finished wall, not the framing
Jamb widths sound like arbitrary numbers until you build them up. A 2x4 is 3-1/2 inches actual. Half inch drywall on both faces adds an inch. That is 4-1/2 inches of finished wall, and the stock jamb sold for it is 4-9/16, a sixteenth wider on purpose so the casing pulls tight against the jamb edge rather than teetering on the board. Same logic on a 2x6: 5-1/2 plus an inch of board is 6-1/2, and the stock jamb is 6-9/16.
| Wall | Finished thickness | Usual stock jamb |
|---|---|---|
| 2x4, 1/2 drywall both faces | 4-1/2" | 4-9/16, 1/16 proud |
| 2x4, 5/8 drywall both faces | 4-3/4" | 4-9/16 is 3/16 short |
| 2x4, 1/2 drywall doubled on one face | 5" | 5-1/4, 1/4 proud |
| 2x4, plaster over lath both faces at 7/8 | 5-1/4" | 5-1/4, exact |
| 2x6, 1/2 drywall both faces | 6-1/2" | 6-9/16, 1/16 proud |
The 5/8 row is the one that catches people, because 5/8 board is common in garages, in party walls and in ceilings that carried down onto the walls, and a prehung unit bought for a standard wall arrives 3/16 short on a wall nobody measured.
Short jambs get extensions, and where you put them matters
When the jamb is narrower than the wall, the fix is a strip of stock ripped to the difference and fastened to the jamb edge so the assembly reaches the wall face. Two choices exist and they are not equivalent. Setting the jamb flush to one face and putting the whole extension on the other gives you one clean side and one joint, which is what most people do when one side is the public side of the wall. Centring the jamb splits the difference and gives two thinner extensions and two joints.
The joint is the reason this is a decision rather than a detail. Every extension has a seam running the height of the opening under the casing, and a seam that lands where the casing does not fully cover it will show through paint after the first heating season. The single-extension approach puts one seam on the side you care about least. Whichever you choose, the extension has to finish flush with the jamb face and be the same thickness, or the casing rocks and you get a gap that opens and closes as the door slams.
Reveal is a setting, not a measurement
The reveal is the strip of jamb edge left visible inside the casing, and it exists so that small inconsistencies in the jamb and casing do not read as errors. Three sixteenths and a quarter inch are both normal. What matters far more than which one you pick is that the same figure is used on both legs and the head of every opening in the room, because the eye reads a reveal that changes from 3/16 to 1/4 across a doorway instantly, and reads a consistent 1/4 everywhere as invisible.
The reveal also eats into the opening as read across the casing edges, which the calculator reports. It does not change the clear passage — the casing sits on the wall face, not in the door swing — but it does change what a plan dimension between casings means.
Walls are not the same thickness top to bottom
Everything above assumes a wall of constant thickness. Real walls bow, studs crown, and a wall that measures 4-1/2 at the floor can measure 4-5/8 at the head where a crowned stud pushed the board out. That is normal, and it is why the extension is ripped a hair wide and planed to fit rather than cut to a number from a calculator.
The practical version: measure the wall at the head, the middle and the floor on both jamb legs. Use the widest reading to size the extension, because you can always trim a proud extension back to the wall with a block plane, and you cannot add material to one that finished narrow. That is six measurements before you cut anything, and it is the difference between a trim job that goes up in one pass and one that comes back off.
What this connects to
The jamb width and the rough opening are separate questions and both have to be right: the rough opening calculator handles the hole in the framing and the shim space around the unit. Once the jambs are in, the trim and molding calculator counts casing footage and sticks for the room, and the drywall calculator covers the board whose thickness you just entered twice. If you are hanging a heavy slab in the jamb you just sized, run the door slab weight calculator before you decide what screws go in the hinges, and check what the finished opening leaves you with the clear width calculator.
One thing not to work out from a calculator: if the wall thickness surprises you because the opening was framed differently from the rest of the wall, find out why before you cut into it. Furred-out walls very often hide plumbing stacks, ducts or electrical runs that were the reason for furring in the first place, and a jamb extension nailed into a supply line is a bad afternoon.
Questions people ask
What jamb width do I need for a 2x6 wall?
For a 2x6 wall with half inch drywall on both faces the finished thickness is 6-1/2 inches, and the stock jamb made for it is 6-9/16, which stands a sixteenth proud on purpose. If the wall has 5/8 board on both faces it is 6-3/4 finished and the 6-9/16 jamb is 3/16 short, so it needs an extension. Measure the actual wall at the opening rather than assuming, because furring, a service cavity or a second layer of board on one side changes the answer and none of it is visible once the wall is finished.
Can I just leave the jamb short and let the casing cover it?
Casing cannot bridge a jamb that sits below the wall face without rocking. The casing lands on the drywall along its outer edge and on nothing along its inner edge, so it pivots, the nails work loose, and a shadow line opens along the jamb that reappears every time the door is closed hard. A sixteenth of an inch is within what caulk and paint absorb. An eighth is visible. Three sixteenths and up wants an extension.
What is the difference between jamb width and jamb depth?
They are the same dimension called two things, and the ambiguity causes real ordering mistakes. The number that varies with the wall — 4-9/16, 5-1/4, 6-9/16 — is how far the jamb reaches from one wall face to the other. Some suppliers list it as width and some as depth. What is never called either is the jamb stock thickness, which is the 3/4 inch or so of material the jamb leg is made from, and that one is nearly always the same regardless of the wall.
The wall measures differently at the top and the bottom. Which number do I use?
Use the widest reading and plan to trim. A jamb or extension that finishes proud of the wall can be planed or sanded back locally where the wall is thin, and the casing will still pull tight. One that finishes narrow at the widest point leaves a gap you cannot close without pulling the trim. Take readings at the head, the middle and the floor on both legs before ripping the extension.
Do I add the flooring thickness to the jamb width?
No. Flooring thickness affects the jamb length and the undercut on the door, not the jamb width. The width runs across the wall from face to face and the floor plays no part in it. What flooring does change is how far the jamb legs have to be cut back at the bottom, and how much gap is left under the slab once the floor goes in, which is a separate calculation covered on the door undercut calculator.