Normally if I working on a database and want perform simple calculations I use the Oracle database rather than starting up the windows calculator.
select 6*1024*1024 from dual; 6*1024*1024 ----------- 6291456
However I recently found out a method of using unix itself (I am sure there are many more options other than this one but I find this quite easy)
man bc gives
bc is an interactive processor for a language that resembles C but provides unlimited-precision arithmetic. It takes input from any files given, then reads the standard input.
In usage/home/oracle $bc 6*1024*1024 6291456 quit
Arithmetic operators are shown in the normal manner + - * / % ^ and there are lots of functions available such as sine, cosine and log, although I would be likely to use a calculator if I wanted to use these.