Mac OS Calculator - How do I keep it on top of Safari WIndow?

I have a MacBook Air mid 2011 1.6 GHz Intel Core i5 running 10.8.5 Mountain Lion.

i also have a mid 2007 iMac with the same OS.

Both have the same issue:

I am on safari and when I open the calculator to try and get additive values from my online bank ledger, I cannot keep the calculator on top of the safari screen; instead it would move the calculator to a new screen and I cannot move/merge the calculator so that I can see and work both claultor and safari together on the same screen.

I have read elsewhere that I would need to use the calculator program under the mac/mac air applications which I am using.

So, what am I doing wrong and how can I use the calculator and keep it on top of the Safari window and use both simultaneously?

Thank you!