International System of Quantities

The system is formally described in a multi-part ISO standard ISO/IEC 80000 (which also defines many other quantities used in science and technology), first completed in 2009 and subsequently revised and expanded.

Within this constraint, the set of base quantities is chosen by convention.

[1] The dimension of a physical quantity does not include magnitude or units.

The conventional symbolic representation of the dimension of a base quantity is a single upper-case letter in roman (upright) sans-serif[c] type.

The following table lists some quantities defined by the ISQ.

The named dimensionless units "radian" (rad) and "steradian" (sr) are acceptable for distinguishing dimensionless quantities of different kind, respectively plane angle and solid angle.

Within the ISQ, all levels are treated as derived quantities of dimension 1.

The ISQ recognizes another logarithmic quantity, information entropy, for which the coherent unit is the natural unit of information (symbol nat).

Working jointly, ISO and IEC have formalized parts of the ISQ by giving information and definitions concerning quantities, systems of quantities, units, quantity and unit symbols, and coherent unit systems, with particular reference to the ISQ.