To measure for a residential storm door, measure the height and width of the inside of the door opening. Take width measurements at the top, bottom and center of the doorway. A residential storm door's size should be 1/4 inch less than the narrowest width measurement.
Storm door sizing charts are available on manufacturer websites such as Larson and Pella. Larson's sizing chart also includes adjustments for doorways that require a shim to adjust the size or depth of the opening. A 1-inch wide by 1-inch deep surface must be clear to mount the storm door hardware. Place shims on the side of the door opening with the hinges to make it slightly deeper. Flat wooden objects such as rulers or paint stirring sticks can be suitable substitutes for shims.
Storm doors protect a home's entrances from harsh weather conditions. For this to work, there must be a tight seal between the door and the edges of the door opening. If the existing door extends past the edge of the trim, remove the trim and attach a layer of wood to make the underlying surface thicker. Attach the trim to the outside of the wooden surface to prepare the opening for a new storm door installation.