At the hardware store I see bifold closet doors listed in sizes like 36x80, 48x80, 72x80. I'm not sure if these are nominal sizes or exactly which parts they're measuring from. How would I determine the correct size of rough opening to create for one of these standard sizes?
The closet doors are most likely listed in actual finished opening size. However it is a good idea to use a tape measure to check 100% for sure. Some doors are made a little oversize to allow trimming into final size for the specified opening size. Other doors may very well be made a small amount undersize to allow for the necessary clearances needed for a finish sized door within the specified opening size.
The installation instructions that come with many doors will be your first go-to source of information regarding the finished opening size is applicable to the doors.
As far as the rough opening size . . . That is totally dependent on your individual situation. Some things that will affect rough opening size are:
= The size and thickness of the door jamb material for cases where a jamb is used.
= Style of jamb used and how the door itself will interact with the jamb.
= Squareness and true of the rough opening. A rough opening that is out of square by some amount will require the jamb to shimmed away from the studs in a way to bring the jamb into square and plumb. This may increase the rough opening size by the amount of shimming that is required.
Bifold rough openings: to the hinge (pivot ) side add 3/4", to the other other side, 1/4"
So for single 24" wide door, a 25" RO (which is really a finished opening for bifolds) is needed.
You need a bit more clearance on the pivot side, because the pivot is inboard a bit and it needs clearance to swing through (pivot around) the thickness of the door. Similarly, any baseboard trim will need to be held back from the inside (of the closet) for door clearance.