millimeters is equal to 25 millimeters. On a ruler, 25 millimeters is equal to 2.5 centimeters. The numbers on a ruler represent centimeters (cm). There are 10 millimeters for each centimeter. To read a metric ruler, each line represents. 10 (1/10) of a centimeter, or 1 millimeter. So, 2.5 millimeters would be between a ruler’s two and 3-centimeter marks. It would be the third line from the zero mark. The 2.5-millimeter mark would be located between the 2 and 3 marks.