Take 3 measurements between opening - Top, middle and bottom - use the smallest measurement.
If posts are already in place, reduce your measurement by 20mm 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”)