Unit: The SI unit of mass is the kilogram, symbol kg.
In the modern SI system, the kilogram is defined by fixing the numerical value of the Planck constant \( h \) to be exactly \( 6.62607015 \times 10^{-34} \) joule·second (J·s). Since \( 1 \,\text{J·s} = 1 \,\text{kg·m}^2\text{s}^{-1} \), this definition links the kilogram to fundamental constants of nature.