Measure the width of the door opening at the top, middle, and bottom
Measure the height of the door opening on the left and right sides
Use the smallest width measurement
Use the smallest height measurement
Measure the door jamb depth from the outside face to the inside stop
Check for any trim, brick molding, or obstructions around the frame
Measure the width of the existing door frame, not the door slab
Measure the height from the threshold to the top of the frame
Verify the storm door swing direction
Record all measurements in inches
Compare measurements to the storm door manufacturer’s size chart