The MIRR function returns the modified internal rate of return for a series of cash flows. The internal rate of return is calculated by using both the cost of the investment and the interest received by reinvesting the cash. The cash flows must occur at regular intervals, but do not have to be the same amounts for each interval.
MIRR(range,finance_rate,reinvestment_rate)
range = range of cells that represent the series of cash flows
finance_rate = interest rate that you pay on the cash flow amounts
reinvestment_rate = interest rate that you receive on the cash flow amounts as they are reinvested