If you plan to replace an existing door, it is best to measure the existing door. Take the tallest side (incl. rebate groove) to determine the height (see fig. 2). You can use the narrow side to determine the width (see fig. 3).

rebated-door-size

If you have no existing doors to measure, then do the following:

Measuring the width of the interior door: start with the minimum measurement that you have taken (see fig. 1). Deduct approximately 10mm from that measurement to determine the correct width of your interior door. 
Measuring the height of the interior door: take the maximum measurement that you have taken (see fig. 2).

Any doubts? Get in touch with a point of sale near you and leave the measuring to an interior door specialist.