FIRSTLY, ALWAYS GO BY THE OLD SAYING "MEASURE TWICE, CUT ONCE!"
Some helpful hints:
Exterior timber doors which are being fitted in an existing frame need to be measured top, middle and bottom: BETWEEN THE EXISTING FRAME LEGS and FROM THE FLOOR (OR THRESHOLD) TO UNDER THE FRAME LINTOL.
Don't measure the existing door as it has probably been fitted badly or cut over previous years. For the new door to fit properly you should always allow a minimum of 2mm each side between the door edges and the frame. In other words: the tight size less 4mm in width, and the same for the height, subject to you reading the next paragraph.
Try and take a look on-line at the Draught excluders from DirectDoors.com for ideas on how best to seal the door against draughts. Some of the threshold draught excluders will require you to cut the door by up to 25mm in height.
As with all things relating to Joinery/Carpentry work common sense has to prevail. If you can't get the exact size of replacement door it may be possible to alter the door frame or the new door. When all else fails www.directdoors.com will offer impartial advice on the most cost effective way to proceed with the last option being the manufacture of a new door.
Be very careful, all joinery tools are sharp and fingers are not easily replaced.
Any questions, email firstname.lastname@example.org
Info supplied by www.DirectDoors.com