Go to the exterior door you wish to install a storm door. Open the exterior door, walk through and turn around to face the door frame.
Take a measurement of the door frame's width in three locations: at the very top, in the middle and at the very bottom. Write down the smallest of these measurements.
Take a measurement of the door frame's height in one location. This measurement can be located anywhere a long the door frame and you will need to measure from the door sweep plate up to the top of the door frame. Write down this measurement.
Calculate the storm door size you need by rounding the width measurement to the nearest whole number and rounding the height measurement up to the next whole number. For example, a storm door that is 30 inches wide by 81 inches tall can have a width measurement ranging from 29 7/8 inches to 30 3/8 inches and a height measurement ranging from 80 inches to 81 inches.