Open the main door fully
Measure the width of the door opening from inside trim to inside trim
Measure the height of the door opening from the top trim to the threshold
Measure the width at the top, middle, and bottom
Measure the height on the left and right sides
Use the smallest width measurement
Use the smallest height measurement
Measure the door frame depth to ensure enough mounting space
Check for obstructions such as handles, locks, or trim
Confirm whether the storm door will mount on the left or right side
Record all measurements in inches
Compare measurements to standard storm door sizes or custom options