The unit of mass is a measure of mass in the International System of Units (SI). The most commonly used unit of mass is the kilogram, which is equal to 1000 grams. Other units of mass include the GRAM (4 letters), SLUG (4 letters) and gramme (6 letters). The GRAM is the SI base unit of mass and is equal to one thousandth of a kilogram. The SLUG is a unit of mass in the Imperial system of units and is equal to 32.17 pounds. The gramme is an obsolete unit of mass that was once used in France and is equal to one thousandth of a kilogram.