Open the existing door and measure the width of the door opening from inside the trim at the top, middle, and bottom
Record the smallest width measurement
Measure the height of the door opening from the top of the threshold to the underside of the top trim on the left, middle, and right
Record the smallest height measurement
Measure the depth of the door frame or trim to ensure there is enough mounting surface for the storm door
Check the hinge side and handle side for any obstructions such as trim, lights, or door hardware
Measure the distance from the edge of the door frame to the existing handle or knob if needed for clearance
Verify whether the door swings left or right and whether it is an inswing or outswing door
Measure the width of the threshold if the storm door will sit on it
Note any irregularities in the opening, such as out-of-square corners or sloped thresholds
Use the smallest width and height measurements when selecting the storm door size
Compare your measurements to the manufacturer’s size chart before purchasing