... , 18, 19, 20, 213/4" x 3/4"19mm x 19mm16, 17, 18, 19, 20, 211" x 1"25.4mm x 25.4mm15, 16, 17, 18 ... 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 ... , 20, 21, 22, 235/8" x 5/8"16mm x 16mm18, 19, 20, 213/4" x 3/4"19.1mm x 19.1mm16, 17, 18, 19 ...