which unit is used to measure mass in the metric system
kilogram
Metric system
 In the modern form of the International System of Units (SI), the seven base units are: "metre" for length, "kilogram" for mass, "second" for time, "ampere" for electric current, "kelvin" for temperature, "candela" for luminous intensity and "mole" for amount of substance. 
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Metric system [SEP] Background
These, together with their derived units, can measure any physical quantity. Derived units may have their own unit name, such as the watt (J/s) and lux (cd/m), or may just be expressed as combinations of base units, such as velocity (m/s) and acceleration (m/s). The metric system was designed to have properties that make it easy to use and widely applicable, including units based on the natural world, decimal ratios, prefixes for multiples and sub-multiples, and a structure of base and derived units. It is also a coherent system, which means that its units do not introduce conversion factors not already present in equations relating quantities.