Measuring the Door Opening
For the width (W), measure the dimension between the right and left exterior trim or door frame at both the top and bottom of the doorway as indicated in the diagram. Record the smallest of the two sizes in the box labeled "Width" below.
For the height (H), measure the inside dimension between the top exterior trim or door frame and the door sill at the lowest point in its slope. Measure in all three locations as indicated in the diagram and record the smallest dimension in the box labeled "Height" below.
Top Trim Mounting Surface
T 
Width (W)
Height (H)
Finally, measure the height of the lowest flat portion of the exterior trim or door frame (T). This must be a minimum of 5/8" (16 mm), and must be completely flat and square.
T |
8
