Look for the placement of door handles, exterior lights, the mailbox and even the doorbell. In some cases these items may interfere with the installation of the storm door or prevent it from closing properly. If this is the case, you may need to relocate them or switch to smaller door handles. Look at the placement of pillars on the front porch to see if the storm door will have enough room to swing outwards once installed. At this point you may also want to decide which way you want your storm door to open. Do you want the handle on the right and the hinges on the left (left-hinged outswing) or the handle on the left and the hinges on the right (right-hinged outswing)?
Place the tape measure on top of the threshold (which is usually made of concrete or silver/brass metal) and stretch it to the bottom side of the top piece of exterior trim. Do this on the left side of the door opening, the center of the opening and on the right side of the opening and make a note of each measurement. Normally, you will get measurements somewhere between 80" to 81" on newer homes and 96" to 97" on openings with older, larger doors. Highlight the smallest of the three measurements, as this is the one you will be working with.
Do this in three places: at the top of the door opening, in the middle of the door opening (around the handle) and at the bottom of the door opening. Make a note of all three measurements. [3] X Research source Highlight the smallest measurement, as this is the one you’ll be using.
For example, if your smallest width measurement was 36 inches (91. 4 cm) and your smallest height measurement was 80 inches (203. 2 cm), then you would write 36" x 80". This is the measurement you will use when purchasing a storm door. If you want to be completely sure that your measurements are accurate, repeat the process a second time.
Choose a storm door manufacturer (such as Larson,[4] X Research source Andersen,[5] X Research source or EMCO) and consult their doorway opening size guide to match your measurements. For example, following the Larson size guide, a door opening measuring 35-7/8" x 80" would require a 36" x 81" standard-size storm door. [6] X Research source
In this situation, it is possible to purchase a z-bar extender kit to fill in the excess space between the door trim and the storm door. This is a convenient option that saves you the trouble of ordering a custom-sized door. However, it is only possible if the door opening is less than an inch wider than the largest standard door size.
This may be more expensive than the cost of standard sized doors, but it will be worth it for a properly fitting storm door. Most of the main storm door manufacturers off a service for ordering custom sized door.
