The first specification to determine is the number of studs/bolts and the ?bolt circle? diameter. Most 4 lug hubs have a 4? bolt circle and most 5 lug boat trailer hubs have a 4-1/2? bolt circle diameter. Cargo and utility trailers with 5 lug hubs are commonly found with a 5? bolt circle. To correctly measure the bolt circle of a hub, measure from the center of the hub to the center of a stud and multiple that measurement by two.