Online Conversion | Metric Measurements

Formal definitions of all SI base units are approved by the CGPM. The first such definition was approved in 1889 and the most recent in 1983. These definitions are modified from time to time as techniques of measurement evolve in order to allow more accurate realizations of the base units.

Current definitions of the base units, as taken from the Comptes Rendus (CR) of the corresponding CGPM, are here shown indented and in a heavy font.

Related decisions which clarify these definitions but are not formally part of them, astaken from the Comptes Rendus (CR) of the corresponding CGPM or the Proce`s-Verbaux (PV) of the CIPM, are also shown indented but in a font of normal weight.

For recent decisions, the appropriate article in Metrologia is also cited. The associated text provides historical notes and explanations but is not part of the definitions themselves.

In accord with the general principles adopted by the ISO (ISO 31), the CIPM recommends that the following rules be observed when using the SI prefixes:
1. Prefix symbols are printed in roman (upright) type with no space between the prefix symbol and the unit symbol.
2. The grouping formed by the prefix symbol attached to the unit symbol constitutes a new inseparable symbol (of a multiple or submultiple of the unit concerned) which can be raised to a positive or negative power and combined with other unit symbols to form compound unit symbols.
Examples :
1 cm3 = (10-2 m)3 = 10-6 m3
1 s-1 = (10-6 s)-1 = 106 s-1
1 V/cm = (1 V)/(10-2m) = 102 V/m
1 cm-1 = (10-2 m)-1 = 102 m-1.
3. Compound prefixes, i.e., prefixes formed by the juxtaposition of two or more SI prefixes, are not used.
Example: 1nm but not 1 mm
4. A prefix is never used in isolation.
Example: 106/m3 but not M/m3.