First things first: It's compound, not compount. In my original version of this page, I mistyped the title, accidentally using a "t" instead of a "d" for the last letter. To my surprise, most of the traffic that this page generated stemmed from people making similar mistakes with their Google search. So — there's the story, and now you know!
For an explanation of how this calculator works, with special attention to compound interest with monthly contributions, have a look at my blog posting on the topic. If your comfortable with the formulas, here's the calculator:
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This is a compound interest calculator that allows for an initial balance and monthly contributions. The calculator assumes that the interest payment is made to your account at the end of the month, and that the interest is therefore compounded on a monthly basis.
Compound Interest Calculator by patrickschneider.com