For the wheel to fit properly, you must know the number of wheel bolts in the hub and bolt circle. To determine the bolt circle on wheels with an even number of bolt holes, you should measure center-to-center on bolt holes directly across from each other on the diameter (example: 8 on 6-1/2 would mean 8-bolt holes with 6-1/2" center-to-center diameter).  An 8-lug trailer wheel should always measure with a spacing of 6.5”.

On odd number bolt wheels, you should measure any bolt hole center to the point halfway between bolt holes on the diameter (example: 5 on 4-1/2 would mean 5-bolt holes with 4-1/2" measure to a point halfway between the opposite bolts).

The tool below is a common one used to measure a 5-bolt trailer wheel.  Not all 5-lug hubs are spaced the same, and this tool measures with ease.  If one is not available, measure from the outside of one bolt hole (or stud) to the center of the one located diagonally across from it, and that measurement will be your bolt pattern in inches.  For example, if you measure 5 inches, then the correct pattern would be a 5 on 5.