How to Launch the Calculator Through Terminal

Always when you have such a question in mind, ie, you don't know the name of the tool to invoke it in command line, find it from the terminal itself:

man -k <keyword>

It will search the manual page names and descriptions for the keyword you specified and prints the titles. In your case, it would be:

man -k calculator

As an illustration, if you want to invoke the webcam tool you can type something like:

man -k camera

man -k is equivalent to apropos.

Qalculate! is awesome tool for desktop.

To install it: sudo apt-get install qalculate.
Terminal version sudo apt-get install qalc.

Converts currency:

 $ qalc 100 dollars to euro
 100 * dollar = approx. EUR 90.017103

 $ qalc 100AUD = x EUR
 (100 * dollar) = (x * euro) = approx. x = 90.017103

Works with percent:

 $ qalc 87.12 + 13.3%
 87.12 + (13.3 * procent) = 87.253

Gets Unix timestamps:

 $ qalc timestamp today
 timestamp("2015-08-14") = 1.4394996E9

Operates boolean:

 $ qalc true != false
 true != false = 1

Also you can do ln -s /usr/bin/qalc /usr/bin/c for shorthand:

 $ c "(2+2)*2"
 (2 + 2) * 2 = 8

The command is gnome-calculator

I created an alias for this in ~/.bash_aliases as follows

calc="gnome-calculator &"

save the file and then type the following in the terminal

source ~/.bash_aliases

You can now launch the calculator using calc in the terminal.

You can try the command below

xcalc