I’m setting up a retirement fund for myself (I’m really planning ahead).
Say I start with $1500 and then add $100 a month at 15% interest, I was just wondering how much money I would have in x amount of years.
I know there are calculators out there that do this but I remember you can do this by geometric progression and would like to do it manually.
Can you give me a hand with the formula?
Well, to find how much all your $100 deposits will add up to, you can use this equation for calculating the future value of an annuity (assuming the interest will be compounded monthly). Future Value =
0.15/12
t = number of years.
Can’t forget the original $1500, so we also need to find how much that will be after t years. Future Value =
1500*(1 + 0.15/12)[sup]12t[/sup]
Add the two together to get the total.
So, for example, after 30 years your annuity will be about $692327.90, your $1500 will have grown to about $131311.48, for a total of $823639.38.
Thank you very much, Cabbage 
The final number is based upon how long you’re keeping it in there and, just as important, how they calculate interest. 15% interest in one lump sum is a lot different than a daily interest that adds up to 15%.