The United States still uses the imperial system of measurements, with a ruler divided into inches. However, most of the world uses the metric system, with a ruler divided into centimeters and millimeters. Becoming accustomed to a different system of measurements may seem like a hassle, but it's well worth the trouble. Fortunately, using and reading a metric ruler is extremely easy.
Things You'll Need
 Metric ruler
 Object to measure

Before you begin, take a moment to study the markings on the ruler. The larger lines denote centimeters (cm). The smaller lines denote millimeters (mm). Note that 1 cm equals 10 mm.

Align the metric ruler next to the object you wish to measure, just as you would a ruler with imperial measurements.

Read the number next to or just prior to the end of your object. The larger line denoting the number of centimeters comes first. For instance, if the end of the object falls at a larger line labeled with the number six, the object is 6 cm.

Count the smaller, or millimeter, markings if applicable. For instance, if the object extends three small markings after the 6 cm point, the object is 6.3 cm long. Alternatively, you may measure the object in millimeters by multiplying the centimeter measurement by 10. So, in this example, you could also say that the object is 63 mm long.
References
 Photo Credit Oregon getting settled: Flickr.com