Annuities are cool. They're fixed payments over a period of time. If you're living in England, there's another financial product called a consol that is an annuity that pays out in perpetuity. Amazing.
Below are simplified examples of how to calculate present value and future value for annuities. I wasn't intending to create a calculator as much as provide a really simple way to think about all the magic Microsoft Excel does behind the scenes.
Theres a lot of room for improvement but I think this is a pretty good starting point.