As of Version 9.0, unit functionality is built into the Wolfram Language
There are many systems of units. The particular set of units that is used depends on factors as various as the field of study and the author's country of origin. The function Convert provides conversion between different units.
convert old to a form involving a combination of newunits
This loads the package.
This converts meters per second into miles per hour.
You have to give prefixes for units as separate words.
The conversion of temperature units is different from most other unit conversions because it is not multiplicative. This is simply because the zeros of various systems are set at different values. For example, zero degrees Centigrade is the same as 32 degrees Fahrenheit.
To organize the use of units, certain standardized systems have been developed. These include the International System (SI) and systems using particular metric units, such as the meter/kilogram/second system. The functions SI, MKS, and CGS convert various units into the standard systems.