Look at the unmarked lines on a metric ruler. There are 2 separate units of measurement on a standard metric ruler—centimeters and millimeters. The numbered lines correspond to centimeters, while the unmarked lines indicate millimeters. If you look closely, you’ll notice that there are 10 millimeters in a centimeter.

Between each centimeter (cm) mark, there should be 10 smaller marks called millimeters (mm). Make sure you read the ruler from left to right. If you are measuring an object, align it with the left side of the zero mark on the ruler. The left side of the line where the object ends will be its measurement in centimeters.

Metric rulers have centimeter and millimeter markings. The larger markings these represent a centimeter. The smaller ticks on a metric ruler represent a millimeter. There are 10 millimeters in a centimeter, so there are 9 millimeter ticks between each centimeter tick.

The center mark between numbers is 0.5cm or 5mm. The larger lines with numbers are centimeters, and the smallest lines are millimeters. Since millimeters are 1/10th of a centimeter, if you measure 8 marks after a centimeter, it is 1.8 centimeters long or 18 millimeters long.