Measure the width of the door opening at the top, middle, and bottom
Use the smallest width measurement
Measure the height of the opening on the left, center, and right
Use the smallest height measurement
Measure the depth of the door frame to ensure enough space for the storm door
Check whether the existing door trim or molding will affect installation
Confirm the door swings in the correct direction
Verify the handle side and hinge side of the existing door
Measure from the outside of the trim if the storm door will mount there
Record all measurements in inches
Compare measurements to the storm door manufacturer’s sizing chart
Ensure the opening is square before ordering
Recheck measurements before purchase