The science and industry are using three major unit systems: Gaussian (CGS), SI and Imperial.
Gaussian (CGS) system is widely used by theoretical and experimental physicists across the world.
SI is engineering standard in Europe and many other countries.
Imperial is engineering standard in UK and USA and few other countries.
Many industries develop the customised units to answer specific industry needs.
Depending on location Petroleum Industry is using few sets of units systems which made of combinations and variations of the major units systems with addition of few non-standard units (see Petroleum Unit Systems).
See Units Conversion for on-line converters between various units.
See also
Natural Science / Engineering / Measurement / Unit
[ Units Conversion ] [ Petroleum Unit Systems ]
Reference
http://www.petroleumoffice.com/unitconverter