The standard unit of mass in the International System of Units (SI) is the kilogram (kg). The kilogram is defined as the mass of the International Prototype of the Kilogram (a platinum-iridium cylinder) stored at the International Bureau of Weights and Measures. It serves as the base unit for mass in the SI system, providing a standard measure used globally in science, industry, and everyday life. Other mass-related units, such as grams (g) and metric tons (t), are derived from the kilogram.