SI - A concise summary of the International System of Units, SI

The last major revision was adopted by the 26th CGPM (2018), which decided that the SI would be based on the fixed numerical values of a set of seven defining constants from which the definitions of the seven base units of the SI would be deduced. [...] By fixing the exact numerical value the unit becomes defined, since the product of the numerical value and the unit has to equal the value of the constant, which is invariant. [...] The language of science: using the SI to express the values of quantities The value of a quantity is written as the product of a number and a unit. [...] When writing the value of a quantity as the product of a numerical value and a unit, both the number and the unit may be treated by the ordinary rules of algebra. [...] It is often useful to use the ratio of a quantity to its unit as the heading for columns in a table, or for labelling the axes of a graph, so that the entries in the table or the labels for the tick marks on the axes are all simply denoted by numbers.