MEASURING THE RIGHT SIZE STORM DOOR
After you have chosen the type of storm door you want, the next step is to measure the door opening and choose the proper size. Now there are a few standard door sizes. For example, a standard door height is anywhere between 80″ inches to 81″ inches. For a taller, 8foot door, the standard height is around 96″ inches. Standard door widths are either 30″ inches, 32″ inches, 34″ inches, or 36″. On a side note, if you see the quotation (“) symbol on a measurement, like 12”, it means “inches”. If you see an apostrophe (‘) on a measurement, like 8′, it means “feet.” If you see an X it means “by”. So a measurement written as 36″ x 8’ would be read as, “Thirtysix inches wide by eight feet tall.”
Now, all storm doors mount to the exterior trim on your entry door. The exterior trim must be wood in order for a proper installation. Oftentimes, the most common type of exterior trim used is called “brickmolding.” Why it’s called brickmolding I have no idea; it’s made of wood. Other times, you may have 1″ x 4″ cedar trim or something similar. No matter, what you are looking for is that there is a flat surface of at least 1″ to mount the storm door frame to and that the depth of the material is at least 1″ as well.
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Once you have determined that you have a proper surface to mount the door to, now it is time to measure the opening.
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Measuring the opening is simple. You want to measure the width of the door from the inside edge of the exterior trim to the inside edge of the exterior trim, and in 3 places to the nearest 1/8″:
Storm Door Measuring Guide

At the TOP,

At the MIDDLE, and

At the BOTTOM of the opening.
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Write down or remember each measurement and make a note of the SMALLEST measurement. Now you are going to measure the height from the threshold to the top, and in 3 places:

On the LEFT side,

In the MIDDLE, and

On the RIGHT side of the opening.
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Once again write down or remember each measurement and make a note
of the SMALLEST measurement.
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Now, write down your two smallest measurements as width X height. If you get confused, write down the smaller of the 2 smallest measurements first; this is the door width. The remaining number of the 2 is the door’s height.
Now you have the actual door opening size. You may refer to the tables below to find out which manufacturer’s door sizes can fit your opening.
And now that you have properly measured your door opening, you are ready for the next step: