The other day, a friend asked me some questions about loans (i.e. how much is the payment for a 15 year vs. a 30 year term, etc...). If he'd known how to use Excel (a popular spreadsheet program), he could have done it himself. Now, thanks to Political Calculations he's got a simple online calcualtor that does the job.
I know there are a lot of other ones online also, but it's worth checking out. He's also got a few other calculators you might want to try.