How long is a toothpick, and how far is it from Dallas to Houston? This 3rd grade metric units of length worksheet asks your child to picture each object before naming a unit. It uses metric units: millimeters, centimeters, meters and kilometers. For each of the five items, two estimates are offered with the same number, and only the unit changes. A fish fork, for example, is about 18 centimeters long, not 18 meters. The second page is an answer key. It ticks the better estimate and says why the other one is too short or too long.