The SI unit of mass is the kilogram, which has the symbol kg. In laboratories, its fraction named as gram ($1 \text{ kg} = 1000 \text{ g}$) is often used due to the smaller amounts of chemicals used in chemical reactions.

The kilogram is defined by taking the fixed numerical value of the Planck constant $h$ to be $6.62607015 \times 10^{-34}$ when expressed in the unit Js, which is equal to $\text{kgm}^2\text{s}^{-1}$.

In this definition, the metre and the second are defined in terms of $c$ and $\Delta V_{Cs}$.