The standard SI unit for mass is the kilogram, denoted as $kg$.

The International System of Units, commonly referred to as SI, which stands for “Système international” in French, represents the modern form of the metric system. It is the most widely adopted system of measurement globally, comprising a coherent framework built on seven base units. Among these, the kilogram serves as the base unit for mass.

Historically, the kilogram was defined as the mass of the International Prototype of the Kilogram (IPK), a cylinder made of platinum-iridium alloy, which is kept at the International Bureau of Weights and Measures in France. However, in 2019, this definition was updated to rely on fundamental constants of nature, particularly the Planck constant. This revision was made to enhance the stability and universality of the unit.

In the realm of physics, mass is a fundamental property of matter. It quantifies an object’s resistance to acceleration when subjected to a net force and also influences the strength of its gravitational attraction to other bodies. The kilogram, as the unit of mass, is essential in various physical calculations, ranging from determining an object’s weight—defined as the gravitational force acting on a mass—to exploring mass-energy equivalence in Einstein’s renowned equation $E=mc^2$.

While the kilogram is the primary unit, other units such as the gram ($g$) and milligram ($mg$) are frequently used, especially when measuring smaller quantities. These derived units are obtained through multiplication or division of the base units. For instance, $1 \, kg$ is equivalent to $1000 \, g$, and $1 \, g$ is equivalent to $1000 \, mg$.

In summary, the kilogram is the standard SI unit for mass and is fundamental to countless calculations and measurements in physics. Its definition, anchored in the Planck constant, ensures its stability and universal applicability across various contexts and scales.