Take 3 measurements between opening - Top, middle and bottom - use the smallest measurement.
If posts are already in place, reduce your measurement by 20mm (4/5") to allow for clearance. Putting in posts, don't forget to also reduce this from your measurement.
When you have your measurement - select from drop down menu. If the size is not there, call or select next size down.
If measuring for double gates reduce your measurement by 25mm (1”)