[amakarevic]

if my door is 28" wide and i haven't bought a jamb yet to fit-measure so to speak, what width should i leave between the studs on each side, if the jamb sizes are standardized ? or is each one different, in which case it would be a good idea to just go on and buy one before measuring the opening.

 

[Willie T]

2-1/4" more than the door size keeps you safe.

 

[Ron6519]

As Willie said, 2 1/4" is fine My general rule is that the rough opening is 2" wider then the door. The door frame, is just three legs you cut to size, it's not a preset package for a 28" door. Measure the wall width to make sure you get the correct depth frame.
Ron

 

[jaros bros.]

Always reduce the room for error as much as possible. You didn't give much information about the door. Go get the jamb and assemble it. Why guess when you can know for sure?