The difference between MIRR and IRR is... The IRR is a more conservative measure of the project's expected rate of return than the MIRR. There is no difference if the project is riskier than the average project that the firm invested in in the past. MIRR assumes that cash flows are reinvestment at the project's payback period, whereas IRR assumes that cash flows are reinvested at WACC MIRR assumes that cash flows are reinvested at the firm's WACC, whereas IRR assumes that cash flows are reinvested at the IRR