It is obviously essential to have the same pattern and number of bolts on both your wheel and wheel hub.  If they are not the same, you won't be able to mount the wheel on your trailer. Different sizes and weight capacities of trailers will have different configurations and they are referred to as bolt patterns.

So if you are replacing either your wheel or hub

A bolt pattern is expressed as the number of bolts by the diameter of the circle formed by the bolts in inches.  The latter is referred to in industry jargon as the Pitch Circle Diameter or PCD for short.   Common bolt patterns are 

• 4 X 4"
• 5 X 4.5"
• 5 X 5.5"
• 6 X 5.5"
• 8 X 6.5"

Only one or two bolt patterns will be associated with an axle of a given weight capacity.  For example, you will never find a 4 X 4" bolt pattern on the hubs of a 10,000 lb. axle.

The first part of the bolt pattern is easy, just count the number of bolts on the hub.  The PCD can be a little more challenging, especially when there is an uneven number of bolts.  In this case, it's not simple to just measure across opposing bolts, because there are none.

Here is an easy calculation you can use to determine the Pitch Circle Diameter:

First, measure the distance ‘S’ between two studs that are adjacent to each other, measuring from the center of one hole to the center of the other.  Then you can use the value you get with this calculation to determine the PCD:

• For wheels/hubs with a 4 Stud configuration, the PCD is equal to S divided by 0.7071
• For wheels/hubs with a 5 Stud configuration, the PCD is equal to S divided by 0.5878
• For wheels/hubs with a 6 Stud configuration, the PCD is equal to S divided by 0.5

On 4, 6, 8, or 10-stud configurations, you can also measure directly across the wheel center between two opposing studs.