The base unit of mass in the International System of Units (SI) is the kilogram. The kilogram is defined as the mass of the international prototype of the kilogram, which is a plum-sized cylindrical artifact made from an alloy of platinum and iridium by Johnson Matthey in 1885. The gram is the basic unit of mass in the metric system. One kilogram is equal to 1000 grams.

In summary, the base unit of mass is the kilogram in the SI system and the gram in the metric system.