Modified Internal Rate of Return
Modified Internal Rate of Return (MIRR) addresses the flaws of standard IRR by explicitly specifying the reinvestment rate for positive cash flows and the financing rate for negative cash flows, producing a single, more realistic measure of an investment's compound annual return.
MIRR was developed specifically to correct the two most serious deficiencies of standard IRR: the unrealistic reinvestment rate assumption and the possibility of multiple solutions. MIRR computes a single rate by separating the treatment of cash inflows and outflows: cash outflows (negative flows) are discounted back to the present at the financing cost (typically WACC), and cash inflows (positive flows) are compounded forward to the end of the project at the reinvestment rate (also commonly WACC, though analysts sometimes use a separate expected return on reinvested cash).
The formula is: MIRR = (FV of positive cash flows at reinvestment rate / PV of negative cash flows at financing rate)^(1/n) - 1, where n is the number of periods. By separating inflows and outflows and using realistic rates for each, MIRR eliminates the multiple-IRR problem entirely and produces a return measure that better reflects real-world economics.
Consider a US real estate development with an initial investment of $10 million (year 0), operating cash inflows of $2 million per year (years 1-5), and a large reclamation/demolition cost of $4 million in year 6 (a non-conventional cash flow profile). Standard IRR might produce two solutions or an unintuitive result. MIRR, using WACC = 8% as both the financing and reinvestment rate, produces a clean, single percentage that accurately reflects the project's economics.
Despite its superiority on technical grounds, MIRR is used far less frequently than standard IRR in US investment practice. Private equity and real estate fund managers report fund-level IRR almost universally, even knowing its limitations, because limited partners are accustomed to the metric and it is embedded in industry databases like Preqin and Cambridge Associates. Corporate capital budgeters often report both IRR and MIRR for unusual cash flow profiles, using MIRR as the definitive measure when the two diverge materially.
The practical lesson for analysts is to use MIRR — or at minimum interpret IRR cautiously — whenever a project has: (1) non-conventional cash flow patterns; (2) large interim inflows that are assumed to be reinvested; or (3) when the standard IRR is unusually high, suggesting the reinvestment rate assumption is particularly unrealistic.