We get this question daily, and the answer is pretty simple. We'll briefly explain it below along with a downloadable template to help.
Match your trailer wheel to the hub by number of bolts and bolt circle.
To determine the bolt circle on the trailer rim with an even number of bolt holes, measure center-to-center on the bolt holes directly across from each other on the diameter (example: 8 on 6.5 would mean 8 bolt holes with 6.5 center-to-center diameter).
On odd number bolt trailer wheels, measure any bolt hole center to the point halfway between bolt holes on the diameter ( Example: 5 on 4.5 would mean 5 bolt holes with 4.5 measure to a point halfway between the opposite bolts).