The kilogram [b] is the base unit of mass in the International System of Units (SI). It is widely used in science, engineering, and commerce worldwide. The kilogram is exactly the mass of one litre of water. As of May 20, 2019, the definition of the kilogram is based on the Planck constant as 6.626 070 15 × 10 −34 kg⋅m 2 ⋅s −1. [1] [2]