There are different steps to find the right coupling head.
Right total weight
The coupling head that you need to choose must have a total weight that corresponds to that of your trailer. The total weight of the coupling head must be equal to, or greater that the total weight of the trailer. You can look at your registration certificate / documentation for your trailer to see its total weight.
Connection tube/drawtube
Coupling heads for braked are bolted on to the drawtube. It is important that you measure the outer diameter of the drawtube and compare this with the inner diameter of the coupling head where the drawtube is to be fitted. Some universal coupling heads have adapters, insert discs so that a heavier coupling head fits a thinner drawtube.
Coupling heads for unbraked trailers are available for both round tubes, and square tubes.
Hole pattern
How does the hole pattern appear, i.e. what is the position of the two bolts the go through the coupling head?
It may be:
Vertical – the bolts run from the top down
Horizontal – the bolts go from side to side
Crossed – both horizontal and vertical bolts
Secondly you need to look at the spacing (c-c) between the bolts.