SI Summary
- The SI is founded on seven SI base units for seven base quantities assumed to be mutually independent.
- Other quantities, called derived quantities, are defined in terms of the seven base quantities via a system of quantity equations. The SI derived units for these derived quantities are obtained from these equations and the seven SI base units.
- The radian and steradian are SI supplementary units which are now interpreted as so-called dimensionless derived units, and they may be used in expressions for SI derived units.
- SI base, derived, and supplementary units may be combined with standard prefixes. In the SDCS we refer to these as scaled SI units.
- Certain units are not part of the International System of Units; that is, they are outside the SI, but are important and widely used. Units in this category are accepted for use with the SI and in the SDCS are referred to as SI associated units.
- Other units outside the SI are currently accepted for use with the SI by NIST. Their continued use is not encouraged. Units in this category are accepted for use with the SI and in the SDCS are referred to as temporary units.