Luckily, yes, there IS a formula for finding the value of annuity investments!

Note:  We are making our investments at the beginning of each
            period (month in most cases.)  The typical annuity formula that
            appears in some Algebra books is for when the investment is
            made at the end of the period.  This causes the first month's
            interest to be lost and isn't representative of how most people

Let's invest $100 each month at 12%*.  How much will we have in one year?

       * If we invest each month, we need to assume that the account
         will be compounded monthly to use this formula.

Again, we'll use the "growth factor" of money.

investment amount = $100

growth factor = $1.01

interest per period = .01

number of periods = 12

Here's the formula:

final amount = investment amount * ( growth factor ^( number of periods ) -1 / interest per period ) * growth factor

With our numbers:

final amount = 100 * ( 1.01 ^( 12 ) -1 / .01 ) * 1.01 = 1280.932804 = $1280.93

Hey, it works!  We got the same amount that we got doing it the long month-by-month way.