... Diameter(BWG)In inchIn metric unit(mm)1/4" x 1/4"6.4mm x 6.4mm22,23,243/8" ... x 3/8"10.6mm x 10.6mm19,20,21,221/2 ... " x 1/2"12.7mm x 12.7mm16,17,18,19 ... /8"16mm x 16mm18,19,20,21,3/4" x 3/4"19.1mm x 19.1mm16,17,18,19 ... ,20,211" x 1/2"25.4mm x 12.7mm16, ... 17,18,19,20,211-1/2" x 1-1/2"38mm x 38mm14,15,16,17,18, ...