A metric chart is a graphical representation listing metric units, and it shows how to convert them into equivalent units within the metric system or within other standard measurement systems. For example, many metric charts list U.S. customary units alongside metric units for ease of converting one into the other.
The three basic metric units are the meter, the gram and the liter. These measure length, mass and volume, respectively. The metric system uses a prefix system that distinguishes between equivalent units by order of magnitude. For example, 1 meter is equivalent to 100 centimeters -- an order of magnitude of two. Metric charts represent this relationship visually by showing the conversion factor needed to make centimeters into meters.
Metric charts also list conversion factors for exchanging metric units into U.S. customary units. U.S. customary units are idiosyncratic, and their conversion factors are less regular, making these metric charts very useful. For example, 1 inch is 22.5 times as long as 1 millimeter, but 1 foot is about one-third of the length of a meter.