The rough opening for a standard sized interior door is 2 inches wider than the state size of the actual door. Doors come in a variety of sizes so a general rule of thumb or practice is considered.
Small inconsistencies can lead to big problems when measuring door openings in the preliminary framing stage. These little difference can make a big impact in the way the doors swings.
A rough opening is commonly framed using two-by-fours. This gives a solid structure for attaching the door in place using hinges. A snug fit is recommended to properly plumb, level and square the door but other details need to be taken into consideration, such as thickness of finished floors. Carpet and thicker tiles also need to be adjusted for.