a value of 3 means 0.3 mm.Most paper specifications will list the thickness of the paper in either mils i.e., thousanths of an inch or mm. If the paper thickness is specified in mm e.g., 0.27 mm, multiply by 10 and round up to the next integer e.g., 0.27 x 10 = 2.7 -> 3. If the paper thickness is specified in mils e.g., 19 mils, multiply by 0.254 and round up to the next higher integer e.g., 19 x 0.254 = 4.826 -> 5. In general, thinner RC papers use a Paper Thickness of 3, thicker fine art matte papers use a Paper Thickness of 5, and the F-type papers use a Paper Thickness of 4.