Measure the width of the door opening at the top, middle, and bottom
Measure the height of the door opening on the left, middle, and right
Use the smallest width measurement
Use the smallest height measurement
Measure the door frame depth if needed for the screen door thickness
Measure from the inside edge of the frame to the opposite inside edge
Check whether the door opening is square by comparing diagonal measurements
Measure the hinge side and latch side if replacing an existing screen door
Measure the existing door itself if you are replacing like for like
Record all measurements in inches or millimeters
Round down to the nearest standard size if required by the manufacturer