Open the existing exterior door and measure the width of the door opening from inside the brickmold or trim, not the door slab
Measure the width at the top, middle, and bottom of the opening
Use the smallest width measurement
Measure the height from the top of the opening to the threshold or sill at the left, center, and right
Use the smallest height measurement
Measure the depth of the trim or brickmold to confirm there is enough mounting surface for the storm door
Check the door swing and confirm whether you need a left-hand or right-hand hinge
Note whether the main door opens inward or outward
Measure any obstructions such as handles, locks, or trim that may affect fit
Compare your measurements to the storm door size chart before purchasing
Round down to the nearest standard storm door size if measurements fall between sizes