The measuring of interior doors is very straight forward.
Have a writing pad to hand if measuring several doors and try and measure each of the doors room by room in a clockwise fashion, it will be easier for you to remember which door goes where.
Measure between the frame at the top, middle and bottom to give yourself a clearer idea as to whether the opening(s) are square; most will not be but don't worry.
DO NOT MEASURE THE DOOR except to assertain the thickness.
In most instances interior doors are 35mm thick, metric doors (which are normally taller than average) are 40mm thick, after all this you then have to decide the best size to suit, normally we at DirectDoors.com advise that you take the widest size for each opening and deduct 4mm from this overall width to allow for ease of fitting.
When you have deducted the 4mm mentioned above this will give you the best size for fitting the door to the narrowest part of the frame width. The only two things to remember are: if you are fitting interior fire doors, firstly they are 45mm thick and secondly they need 3 fire rated hinges.
I hope this advice is helpful.
Any questions, email firstname.lastname@example.org
Info supplied by www.directdoors.com