Below you'll find information on how to measure both a door and window liners.  Using these directions will take alot of the pain of getting the exact fit.

DOORS:  

> MEASURE DOOR SLAB WIDTH AND HEIGHT
Measure door width on the side where the hinge pins show (because the door edge at the strike edge is beveled)

> MEASURE ROUGH OPENING WIDTH AND HEIGHT IF POSSIBLE
Width should be 2” wider than the door size, for example a 2’ 6” door, the width should be 2’ 8”, or 32”. Height should be 2 ½” higher than the door height for interior doors, and 3” higher than the door height for exteriors.

> RECORD THE DOOR SWING HANDING
Viewing the door from the side where you see the hinge pins

--RIGHT HAND DOOR if the hinge pins are on your right

--LEFT HAND DOOR if the hinge pins are on your left

WINDOW LINERS:

> Measure the outside dimensions of the actual window frame, vinyl or aluminum.  This measure would be about ½” less than the actual rough opening width and height. This width and height will be the outside measure of your window liner frame, or the window jamb frame, and so is the WIDTH AND HEIGHT.

> Measure the distance from the window frame in to the wall surface.

> Measure all sides, because some times this can vary.  This distance is your JAMB, OR LINER, DEPTH.  

If you wish, you can order the depth a little wider to allow for custom trimming to width on the jobsite.