The IRR (Internal Rate of Return) formula in Excel is: IRR(range of cash flows, [guess]) Where range of cash flows: The range of cells containing cash flows, including the initial investment. These should be in chronological order. guess (optional): An estimate of the IRR. It's not required, but you can provide it for quicker calculation convergence. The IRR function calculates the rate at which the net present value (NPV) of cash flows becomes zero, indicating the project's internal rate of return.