section_path: Definition/Prefixes
section_name: Prefixes
paragraph_text: Prefixes
  
  Like all metric systems, the SI uses metric prefixes to systematically construct, for the same physical quantity, a set of units that are decimal multiples of each other over a wide range. For example, driving distances are normally given in kilometres (symbol km) rather than in metres. Here the metric prefix ' kilo- ' (symbol 'k') stands for a factor of 1000; thus, 1 km = 1000 m.
  
  The SI provides twenty-four metric prefixes that signify decimal powers ranging from 10 to 10, the most recent being adopted in 2022. Most prefixes correspond to integer powers of 1000; the only ones that do not are those for 10, 1/10, 100, and 1/100.The conversion between different SI units for one and the same physical quantity is always through a power of ten. This is why the SI (and metric systems more generally) are called decimal systems of measurement units.
  
  The grouping formed by a prefix symbol attached to a unit symbol (e.g. ' km ', ' cm ') constitutes a new inseparable unit symbol. This new symbol can be raised to a positive or negative power. It can also be combined with other unit symbols to form compound unit symbols. For example, g/cm is an SI unit of density, where cm is to be interpreted as (cm).
  
  Prefixes are added to unit names to produce multiples and submultiples of the original unit. All of these are integer powers of ten, and above a hundred or below a hundredth all are integer powers of a thousand. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth, so there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined, so for example a millionth of a metre is a micrometre, not a millimillimetre. Multiples of the kilogram are named as if the gram were the base unit, so a millionth of a kilogram is a milligram, not a microkilogram.
  
  The BIPM specifies 24 prefixes for the International System of Units (SI):