Show the value of your investment idea with IRR If you've ever wanted to show your manager how an investment idea will pay off over time, Excel's Internal Rate of Return (IRR) functoin is here to help. This example calculates IRR by considering an initial investment and the projected values of the cash flows created by that investment over the next 5 years. Note that Excel assumes that periods are measurng years. IRR can be calculated based on monthly cash flows by dividing IRR by 12, or by usin