Measure the length and width of the door frame, making sure to measure from end to end inside the door frame.
Remove any casing that is inside the door frame. Measure the length and width again, making sure to line your measuring tape up with the studs on each side or end.
Measure across the length of the top and bottom of the door frame, making sure to keep the measuring tape inside the frame.
Measure the length from the right top corner to the right bottom corner. Then measure the distance between the left top corner and the left bottom corner, making sure to keep the measuring tape inside the door frame.