PPF is a very cunning invenstment plan and if you dont read between the line then you will never make the money you thought you will, 15 years from now. The trick in PPF investment (which can be either a BANK or post office) is to invest how much you plan to invest for that financial year in one shot. If one shot is not possible then you can try with 2 months or so but not later. The thing with PPF is that the interest is calculated on 5th of every month against the "Lowest balance" on that date. So if you dont really think it through there is a good chance you will lose money.