The rough door frame opening is the one formed by the two upright studs on the sides -- called trimmers -- and the horizontal header that spans the top. The size of this opening is critical. If it's slightly too small, your door may fit, but it will stick and, eventually, you may not even be able to close it. If the opening is too wide, on the other hand, the door won't stay closed, and it will be drafty. If you have a busy family, you want a door that just works, and that calls for measuring the opening properly.
Measure the width of the door you plan to install with a tape measure. Standard sizes include 36, 32, 30 and 28 inches. Yours may not be standard, though, and may require a custom opening.
Add twice the thickness of the door jamb material -- which is almost always 3/4 inches -- to the door width measurement. Add to this an extra 1/2 inch for what builders like to call "wiggle room" to arrive at the width of the rough opening. For example, if you have a 30-inch door and 3/4-inch jamb material, you need a 32-inch-wide opening.
Calculate the height of the opening by measuring the door and adding the thickness of the top jamb. If you're installing the door on a bare wood subfloor, you also need to allow for the thickness of the floor covering or threshold as well as gaps above and below the door. In most cases, this simplifies to a simple formula: Make the rough opening 2 1/2 inches higher than the door.