To invoke units for interactive use, type `units` at your shell
prompt. The program will print something like this:

Currency exchange rates from www.timegenie.com on 2014-03-05 2860 units, 109 prefixes, 85 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 units you want to convert
*to*. To convert to feet, you would type `feet`. If the
`readline` library was compiled in then <tab> will
complete unit names. See Readline Support, for more information
about `readline`. To quit the program under Unix, press
<Ctrl-C> or <Ctrl-D>. Under Windows, press <Ctrl-C> or
<Ctrl-Z>; with the latter, you may also need to press <Enter>.

The result 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 0.03.

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 0.03048 dekameters.
But the number given the other direction is inexact.

If you 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 that 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 <Enter> 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 units 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

Named dimensionless units are not treated as dimensionless
in other contexts. They cannot be used as exponents
so for example, ‘`meter^radian`’ is not allowed.
If you want a list of options you can type `?` at the
‘`You want:`’ prompt. The program will display a list of named
units that are conformable with the unit that you entered at the
‘`You have:`’ prompt above. 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 historical information about the unit. (You
can generally quit out of the page by pressing ‘`q`’.)

Typing `search` `text` will display a list of all of the units
whose names contain `text` as a substring along with their definitions.
This may help in the case where you aren't sure of the right unit name.