2-1/2 inch diameter base with 1/4-20 inch stud. C Size Ball 1-1/2 inch diameter. There are 7 holes in the base; 4 holes in a standard AMPS and 3 holes 120 degrees apart 1-4-20 stud Works with standard Camera/Camcorder threaded holes. For use with RAM universal ball and socket mount system. Marine grade aluminum with powder coat finish and rubber ball. Lifetime Warranty.