# Units (skhep.units)¶

Subpackage for the HEP System of Units, (derived) physical units and commonly-used unit prefixes.

The HEP System of Units is the set of basic units originally defined by the CLHEP project.

## Common unit prefixes¶

There are 2 types of prefixes:

- SI prefixes [SI_prefixes].
- Binary prefixes [Binary_prefixes].

**References**

[SI_prefixes] | https://en.wikipedia.org/wiki/International_System_of_Units#Prefixes. |

[Binary_prefixes] | https://en.wikipedia.org/wiki/Unit_prefix#Binary_prefixes. |

## Module of HEP basic units¶

In HEP the System of Units consists of the basic units originally defined by the [CLHEP] project:

Quantity Name Unit Length millimeter mm Time nanosecond ns Energy Mega electron Volt MeV Positron charge eplus Temperature kelvin K Amount of substance mole mol Luminous intensity candela cd Plane angle radian rad Solid angle steradian sr

It is largely based on the international system of units ([SI])

Quantity Name Unit Length meter m Time second s Mass kilogram kg Electric current ampere A Temperature kelvin K Amount of substance mole mol Luminous intensity candela cd

but augments it with handy definitions, changing the basic length and time units.

This module also defines an extensive set of derived units.

Typical use cases:

```
>>> # add two quantities with length units and get the result im meters
>>> from skhep import units as u
>>> (1 * u.meter + 5 * u.cm) / u.meter
1.05
>>> from skhep.units import MeV, GeV
>>> massWindow = 100 * MeV # define a 100 MeV mass window
>>> def energy_resolution():
... # returns the energy resolution of 100 MeV
... return 100 * MeV
...
>>> energy_resolution() / GeV # get the energy resolution in GeV
0.1
```

**References**

[CLHEP] | http://proj-clhep.web.cern.ch/proj-clhep/. |

[SI] | http://www.physics.nist.gov/cuu/Units/index.html. |