What size door jamb would I need
the door frame opening is 31 3/4 and I have a 30 inch door?
2×4 framing with 1/2 inch drywall
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Well, if the framing is perfectly plumb, you should be able to make this work.
Newer door slabs are usually beveled on edge(s) and the long width is also usually an eighth shy of called size.
If you bought a prehung and measured the exact at header and that would fit in the perfectly plumb opening, then you should be able to fit it to work.
Here side jambs are available rabbited with the head pc cut long. Do the math and see what you get. Cut the head to fit in the rabbits plus your measured door width plus at least a qtr inch. Mortise the hinges to the jamb and door. Hang door to the one side jamb. Slip the head into the rabbits and fit the strike jamb to it. The gap at the head is what you’ll get at the bottom when installed.
Is the gap too big at the top? If so, shorten the head pc.
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Now, will the door with jamb conform to your opening? If so, then you set the jamb, hang the door, drill for the knobset, mortise the strike and trim out.
If your work above the line shows you can’t get it in the perfectly plumb opening, you’ll have to trim the latch side of the door to work and continue.
Most people would like at least a 3/4” bigger rough opening at least to hang that door.......
You don’t have that. Best of luck!
So would it be best to get a smaller door?
Not necessarily, I’ve used everything including a shoehorn to make something work.
Depending on the construction of the door it’s possible to reduce the width of the jamb and narrow the slab to get what you need to work. In standard width, the next narrowest prehung or slab would be 2’-4”. If your trim etc would work, that would be an option.
I think you're asking about the width of the jamb not the thickness. A 30" door should be fine if things are reasonably plumb. The jamb should be 3.5" + .5" + .5" + 1/16" = 4 9/16". This is a standard jamb.
Standard jamb thickness is 5/8".