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`units`

The `units`

program converts quantities expressed in various
systems of measurement to their equivalents in other systems of
measurement.
Like many similar programs, it can handle multiplicative scale changes.
It can also handle nonlinear conversions such as Fahrenheit to
Celsius;^{1}
see Temperature Conversions.
The program can also perform conversions from and to sums of
units, such as converting between meters and feet plus inches.

Basic operation is simple: you enter the units that you want to convert
*from* and the units that you want to convert *to*.
You can use the program interactively with prompts, or you can use it
from the command line.

Beyond simple unit conversions, `units`

can be used as a
general-purpose scientific calculator that keeps track of units in its
calculations. You can form arbitrary complex mathematical expressions
of dimensions including sums, products, quotients, powers, and even
roots of dimensions. Thus you can ensure accuracy and dimensional
consistency when working with long expressions that involve many
different units that may combine in complex ways; for an illustration,
see Complicated Unit Expressions.

The units are defined in several external data files. You can use the extensive data files that come with the program, or you can provide your own data file to suit your needs. You can also use your own data file to supplement the standard data files.

You can change the default behavior of `units`

with various
options given on the command line. See Invoking `units`

, for a
description of the available options.

But Fahrenheit to
Celsius is linear, you insist. Not so. A transformation *\(T\)* is linear if
*\(T(x+y)=T(x)+T(y)\)* and this fails for *\(T(x)=ax+b\)*. This transformation is
affine, but not linear—see https://en.wikipedia.org/wiki/Linear_map.

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