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?