The unit of metric system is composed of three parts: the METRE (5 letters), the GRAM (4 letters) and the METER (5 letters). The METRE is a unit of length in the metric system, equal to 100 centimeters or approximately 39.37 inches. The GRAM is a unit of mass in the metric system, equal to one thousandth of a kilogram. The METER is a unit of volume in the metric system, equal to the volume of a cube with sides of one METRE in length.