These premium quality heavy duty double flange stainless steel spring bars are 1.5 mm thick for sizes 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 and 1.8 mm thick for sizes 18, 19, 20, 21, 22, 23, 24, 26, 28, 30, 32, 34, 36, 38 mm. For only size 18, 19, 20 and 21mm, we put 4 extra pieces of 1.5 mm spring bars along with 6 pieces of 1.8mm spring bars. Size can be determined by measuring width of the case lugs where watch band is installed. See picture showing where to measure size in mm (millimeter).