Before you install a door in a door frame, you need to make sure the door will actually open -- you may have furniture or other obstructions in the door's way. Calculating the swing of the door and determining how much room it needs to open is a relatively simple task that uses basic math. Consider that the hinge of the door is like the center of a circle, and the door its radius -- the door will always be the same size, even when it moves, so you must simply measure, and mark the area accordingly to check your measurements.
Things You'll Need
Measure the width of the door before you install it in the door frame.
Place the end of your tape measure against the door frame on the side where the door will connect. The tape measure should be at floor level and locked in place at the measurement of the door's width. Start by running the tape straight across the threshold. Tear off a piece of masking tape, and stick it to the floor at the end of the measuring tape opposite the spot it is connected to the door frame.
Keeping the end of the tape measure in place against the door frame and the tape at floor level, swing out the other end -- the end you used to mark the floor. Swing it out 45 degrees, and mark the floor by tearing off another piece of masking tape and sticking it down.
Swing the tape out again another 45 degrees, and mark the floor again. You should now have three pieces of tape on the floor -- one at 0 degrees (on the threshold), one around 45 degrees and one at 90 degrees. One end of the measuring tape should still be anchored against the door frame. Swing the other end -- the one in your hand, that you used to mark the floor -- to the halfway point between the 45-degree mark and the 90-degree mark. Mark the floor with tape.
Swing the tape to the halfway point between the 0-degree mark and the 45-degree mark. Mark the floor. You should now have five tape dots on your floor. Tear off strips of masking tape to connect the dots. This shows you the arc of the swing of your door, and enables you to see how much room the door needs to open.