* doc/misc/calc.texi (Basic Operations on Units):

  (Customizing Calc): Mention the variable `calc-allow-units-as-numbers'.

* lisp/calc/calc.el (calc-allow-units-as-numbers): New variable.
* lisp/calc/calc-units.el (calc-convert-units): Use new variable.

1 files changed, 27 insertions, 9 deletions

diff --git a/doc/misc/calc.texi b/doc/misc/calc.texi
index a5514867a99..da018acffe9 100644
--- a/doc/misc/calc.texi
+++ b/doc/misc/calc.texi
@@ -27866,11 +27866,6 @@ acres per meter-second.)  Remainder units are expressed in terms of
 ``fundamental'' units like @samp{m} and @samp{s}, regardless of the
 input units.
 
-If you want to disallow using inconsistent units, you can set the customizable variable
-@code{calc-ensure-consistent-units} to @code{t} (@pxref{Customizing Calc}).  In this case,
-if you request units which are inconsistent with the original units, you will be warned about
-it and no conversion will occur.
-
 One special exception is that if you specify a single unit name, and
 a compatible unit appears somewhere in the units expression, then
 that compatible unit will be converted to the new unit and the
@@ -27880,6 +27875,12 @@ change the @samp{s} to @samp{ms} to get @samp{9.8e-4 cm/ms^2}.
 The ``remainder unit'' @samp{cm} is left alone rather than being
 changed to the base unit @samp{m}.
 
+If you want to disallow using inconsistent units, you can set the
+customizable variable @code{calc-ensure-consistent-units} to @code{t}
+(@pxref{Customizing Calc}).  In this case, if you request units which
+are inconsistent with the original units, you will be warned about it
+and no conversion will occur.
+
 You can use explicit unit conversion instead of the @kbd{u s} command
 to gain more control over the units of the result of an expression.
 For example, given @samp{5 m + 23 mm}, you can type @kbd{u c m} or
@@ -27912,10 +27913,16 @@ Composite units are expanded as if by @kbd{a x}, so that
 
 If the value on the stack does not contain any units, @kbd{u c} will
 prompt first for the old units which this value should be considered
-to have, then for the new units.  Assuming the old and new units you
-give are consistent with each other, the result also will not contain
-any units.  For example, @kbd{@w{u c} cm @key{RET} in @key{RET}}
-converts the number 2 on the stack to 5.08.
+to have, then for the new units.  (If the value on the stack can be
+simplified so that it doesn't contain any units, like @samp{ft/in} can
+be simplified to 12, then @kbd{u c} will still prompt for both old
+units and new units. You can ignore the prompt for old units with
+@key{RET}, or turn off the prompt by setting the customizable variable
+@code{calc-allow-units-as-numbers} to @code{nil}. 
+@pxref{Customizing Calc})  Assuming the old and new units you give are
+consistent with each other, the result also will not contain any
+units.  For example, @kbd{@w{u c} cm @key{RET} in @key{RET}} converts
+the number 2 on the stack to 5.08.
 
 @kindex u b
 @pindex calc-base-units
@@ -35694,6 +35701,17 @@ have different dimensions. The default value of @code{calc-ensure-consistent-uni
 is @code{nil}.
 @end defvar
 
+@defvar calc-allow-units-as-numbers
+When converting units, the variable @code{calc-allow-units-as-numbers}
+determines whether or not values which can be simplified so that
+they don't contain units (such as @samp{ft/in} can be simplified to 12)
+can be regarded as not containing units.  If 
+@code{calc-allow-units-as-numbers} is non-@code{nil}, then @kbd{u c}
+will prompt for both old units and new units when converting an expression
+like @samp{ft/in}, otherwise @kbd{u c} will only prompt for the new units.
+The default value of @code{calc-allow-units-as-numbers} is @code{t}.
+@end defvar
+
 @defvar calc-undo-length
 The variable @code{calc-undo-length} determines the number of undo
 steps that Calc will keep track of when @code{calc-quit} is called.