Go here.There are heaps of webpages with formulas for calculating compounding interest, but none (that I have found) that explains the formula to use when the deposit frequency does NOT necessarily match the compounding frequency.

In other words:

§1: equal sized deposits are made regularly at the beginning of each month

§2: compounding of interest m times a year

§3: interest rate i

§4: t months to grow

What is the value of the principal at the end of each MONTH?