Measuring up for a field (5 bar) gate is not quite as straightforward as it seems and can be a little tricky.
This is further complicated as there are no standard sizes for fittings.  The dimensions of the various styles and types of fittings can vary between suppliers. The dimensions shown below are for the fittings we currently supply, so if you use other fittings the dimensions may need to be adjusted.

The infographic below shows the way to calculate the width of the gates required for existing posts or the distance between the posts to suit a particular size gate.
If you currently just have a gap & wish to fit a 5 bar gate, then in conjunction with the minimum post size guide shown in brown against every gate on the 5 Bar Field & Driveway Gates page you can work out what size of the gates you need.