The most commonly used quick reference is the amortization formula for figuring out the repayment on a loan given the criteria- typically your home or vehicle finance.
However, sometimes you also want to work it backwards and forwards a little, finding out the maximum loan, for example, given a monthly repayment and interest rate.
MortyJs, the lighter cousin to Morty and MortyPy is just the reference you need.
Note:The numbers will jump around a little due to rounding errors, but only by cents- not rands. But it's close enough to work with and get an idea.

Download Morty Javascript library
Download (right-click 'save as') unit tests for JsUnit