# (units.info)Interactive use

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Enter node , (file) or (file)node
```
2 Interacting with `units'
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To invoke units for interactive use, type `units' at your shell prompt.
The program will print something like this:

2131 units, 53 prefixes, 24 nonlinear units

You have:

At the `You have:' prompt, type the quantity and units that you are
converting _from_.  For example, if you want to convert ten meters to
feet, type `10 meters'.  Next, `units' will print `You want:'.  You
should type the type of units you want to convert _to_.  To convert to
feet, you would type `feet'.  Note that if the readline library was
compiled in then the tab key can be used to complete unit names.  Note:

The answer will be displayed in two ways.  The first line of output,
which is marked with a `*' to indicate multiplication, gives the result
of the conversion you have asked for.  The second line of output, which
is marked with a `/' to indicate division, gives the inverse of the
conversion factor.  If you convert 10 meters to feet, `units' will print

* 32.808399
/ 0.03048

which tells you that 10 meters equals about 32.8 feet.  The second
number gives the conversion in the opposite direction.  In this case,
it tells you that 1 foot is equal to about 0.03 dekameters since the
dekameter is 10 meters.  It also tells you that 1/32.8 is about .03.

The `units' program prints the inverse because sometimes it is a
more convenient number.  In the example above, for example, the inverse
value is an exact conversion: a foot is exactly .03048 dekameters.  But
the number given the other direction is inexact.

If you try to convert grains to pounds, you will see the following:

You have: grains
You want: pounds
* 0.00014285714
/ 7000

From the second line of the output you can immediately see that a grain
is equal to a seven thousandth of a pound.  This is not so obvious from
the first line of the output.  If you find  the output format
confusing, try using the `--verbose' option:

You have: grain
You want: aeginamina
grain = 0.00010416667 aeginamina
grain = (1 / 9600) aeginamina

If you request a conversion between units which measure reciprocal
dimensions, then `units' will display the conversion results with an
extra note indicating that reciprocal conversion has been done:

You have: 6 ohms
You want: siemens
reciprocal conversion
* 0.16666667
/ 6

Reciprocal conversion can be suppressed by using the `--strict'
option.  As usual, use the `--verbose' option to get more
comprehensible output:

You have: tex
You want: typp
reciprocal conversion
1 / tex = 496.05465 typp
1 / tex = (1 / 0.0020159069) typp

You have: 20 mph
You want: sec/mile
reciprocal conversion
1 / 20 mph = 180 sec/mile
1 / 20 mph = (1 / 0.0055555556) sec/mile

If you enter incompatible unit types, the `units' program will print
a message indicating that the units are not conformable and it will
display the reduced form for each unit:

You have: ergs/hour
You want: fathoms kg^2 / day
conformability error
2.7777778e-11 kg m^2 / sec^3
2.1166667e-05 kg^2 m / sec

If you only want to find the reduced form or definition of a unit,
simply press return at the `You want:' prompt.  Here is an example:

You have: jansky
You want:
Definition: fluxunit = 1e-26 W/m^2 Hz = 1e-26 kg / s^2

The output from `units' indicates that the jansky is defined to be
equal to a fluxunit which in turn is defined to be a certain combination
of watts, meters, and hertz.  The fully reduced (and in this case
somewhat more cryptic) form appears on the far right.

Some named units are treated as dimensionless in some situations.
These include the radian and steradian.  These units will be treated as
equal to 1 in units conversions.  Power is equal to torque times
angular velocity.  This conversion can only be performed if the radian
is dimensionless.

You have: (14 ft lbf) (12 radians/sec)
You want: watts
* 227.77742
/ 0.0043902509

Note that named dimensionaless units are not treated as dimensionless
in other contexts.  They cannot be used as exponents so for example,

If you want a list of options you can type `?' at the `You want:'
prompt.  The program will display a list of named units which are
conformable with the unit that you entered at the `You have:' prompt
above.  Note that conformable unit combinations will not appear on this
list.

Typing `help' at either prompt displays a short help message.  You
can also type `help' followed by a unit name.  This will invoke a pager
on the units data base at the point where that unit is defined.  You
can read the definition and comments that may give more details or