The metric conversion ladder is the ordered list of metric units from greatest to least with the standard unit of measurement in the middle. The ladder is often used as a mnemonic or educational device to assist in converting between metric units.
There are three standard units of measurement: the meter for length, the liter for volume and the gram for weight. However, there are many instances of objects being too large or too small to be described using the standard unit. Therefore, there are six commonly used prefixes that, when added to the standard metric unit, describe a much larger or smaller unit. In descending order, these are kilo, hecto, deka, deci, centi and milli. For example, a kilometer is the equivalent of 1000 meters, a dekagram is the equivalent of 10 grams and a milliliter is the equivalent of .001 liters.
Since these units are all defined with respect to the standard metric unit, conversion is made much simpler. For example, a milliliter is defined as 10^-3 liters, a dekagram is defined as 10^1 grams, and a kilometer is defined as 10^3 meters. Due to this simple conversion, many science and math classes use the metric system instead of the United States customary units of pounds, inches and fluid ounces.