As of 2014, dimes have the smallest diameter of any U.S. coin and are also the thinnest. A penny is 1.52 mm thick with a diameter of 19.05 millimeters. It weighs 2.5 grams and has plain edges. A nickel is 1.95 millimeters thick with a diameter of 21.21 millimeters. It weighs 5 grams and has plain edges. A quarter is 1.75 millimeters thick and has a diameter of 24.26 millimeters.