Calc provides ways to get quick access to your selected "favorite" units, as well as ways to define your own new units.
To select your favorite units, store a vector of unit names or
expressions in the Calc variable Units. The u 1
through u 9 commands (calc-quick-units) provide access
to these units. If the value on the top of the stack is a plain
number (with no units attached), then u 1 gives it the
specified units. (Basically, it multiplies the number by the
first item in the Units vector.) If the number on the
stack does have units, then u 1 converts that number
to the new units. For example, suppose the vector `[in, ft]'
is stored in Units. Then 30 u 1 will create the
expression `30 in', and u 2 will convert that expression
to `2.5 ft'.
The u 0 command accesses the tenth element of Units.
Only ten quick units may be defined at a time. If the Units
variable has no stored value (the default), or if its value is not
a vector, then the quick-units commands will not function. The
s U command is a convenient way to edit the Units
variable; see section Other Operations on Variables.
The u d (calc-define-unit) command records the units
expression on the top of the stack as the definition for a new,
user-defined unit. For example, putting `16.5 ft' on the stack and
typing u d rod defines the new unit `rod' to be equivalent to
16.5 feet. The unit conversion and simplification commands will now
treat rod just like any other unit of length. You will also be
prompted for an optional English description of the unit, which will
appear in the Units Table.
The u u (calc-undefine-unit) command removes a user-defined
unit. It is not possible to remove one of the predefined units,
however.
If you define a unit with an existing unit name, your new definition will replace the original definition of that unit. If the unit was a predefined unit, the old definition will not be replaced, only "shadowed." The built-in definition will reappear if you later use u u to remove the shadowing definition.
To create a new fundamental unit, use either 1 or the unit name itself as the defining expression. Otherwise the expression can involve any other units that you like (except for composite units like `mfi'). You can create a new composite unit with a sum of other units as the defining expression. The next unit operation like u c or u v will rebuild the internal unit table incorporating your modifications. Note that erroneous definitions (such as two units defined in terms of each other) will not be detected until the unit table is next rebuilt; u v is a convenient way to force this to happen.
Temperature units are treated specially inside the Calculator; it is not possible to create user-defined temperature units.
The u p (calc-permanent-units) command stores the user-defined
units in your `.emacs' file, so that the units will still be
available in subsequent Emacs sessions. If there was already a set of
user-defined units in your `.emacs' file, it is replaced by the
new set. (See section General Mode Commands, for a way to tell Calc to use
a different file instead of `.emacs'.)