Convert Inches To Millimeters (in to mm) Calculator Published April 10, 2018 Updated January 19, 2022 » Skip down to calculator Convert Inches (in) to Millimeters (mm) using this conversion calculator. 1 inch is equal to 25.4 milliimeters. If you need to know how to convert inches to millimeters, the formula is to multiply inches by 25.4.