How to Measure Your Doors
The best and easiest way to measure your door is to
Open the door
Then place your tape on one edge making sure it stays in place (you could get someone to help with this)
Carry the other end of the tape to the other edge of the door
Finally take a reading from the tape
If you can't measure the door you can measure the frame, its the same principle but you just measure where the door sits in the frame. From where the hinges are to the keep plate for the latch is.
Below is a chart with all door sizes, I know from experience that most household doors don't exactly match the sizes below but they should be relativey close.