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Modified IRR Calculator

The modified internal rate of return (MIRR) addresses a fundamental limitation of standard IRR by incorporating realistic assumptions about how cash flows are actually deployed. Unlike IRR, which assumes reinvestment at the project rate, MIRR lets you specify separate financing and reinvestment rates—reflecting market conditions and your actual capital constraints. This is essential for comparing capital-intensive projects where reinvestment assumptions significantly impact returns.

Last updated:

Creators Arturo Barrantes, MSc Finance
7,669 people find this calculator helpful

Understanding Modified Internal Rate of Return

MIRR measures the annualized return on an investment after accounting for the cost of financing negative cash flows and the returns earned on reinvested positive cash flows. It bridges the gap between IRR's theoretical assumptions and real-world capital markets.

Standard IRR implicitly assumes all intermediate cash inflows are reinvested at the project's own IRR—a rate that may be unrealistically high or inconsistent with market conditions. MIRR corrects this by letting you specify:

  • Financing rate: The cost of borrowing to cover negative cash flows (typically your cost of debt or cost of capital)
  • Reinvestment rate: The rate earned when you reinvest positive cash flows (often your cost of capital or expected market return)

This makes MIRR more conservative and often more realistic than IRR for decisions about long-term projects, capital budgets, and equipment purchases.

MIRR Formula

The MIRR calculation unfolds in three steps: discount all negative cash flows to present value using the financing rate, compound all positive cash flows to future value using the reinvestment rate, then solve for the rate that equates them over the project lifetime.

MIRR = (FV(C₊, RR) ÷ PV(C₋, FR))^(1/n) − 1

  • FV(C₊, RR) — Future value of positive cash flows compounded at the reinvestment rate
  • PV(C₋, FR) — Present value of negative cash flows discounted at the financing rate
  • n — Total number of periods (years) in the project timeline
  • RR — Reinvestment rate (annual percentage applied to positive flows)
  • FR — Financing rate (annual percentage applied to negative flows)

MIRR Calculation Example

Suppose you evaluate a 5-year project with these cash flows:

  • Initial investment: −$10,000
  • Year 1: +$6,000
  • Year 2: −$4,000
  • Year 3: +$8,000
  • Year 4: +$3,000
  • Year 5: +$7,000

Assume a financing rate of 10% and reinvestment rate of 12%.

Step 1: Future value of positive flows

FV = 6,000(1.12)⁴ + 8,000(1.12)² + 3,000(1.12) + 7,000 = $29,836

Step 2: Present value of negative flows

PV = 10,000 + 4,000/(1.10)² = $13,306

Step 3: Solve for MIRR

MIRR = ($29,836 ÷ $13,306)^(1/5) − 1 ≈ 17.87%

Common Pitfalls When Calculating MIRR

MIRR is more robust than IRR, but several mistakes can skew results significantly.

  1. Confusing financing rate with discount rate — The financing rate applies only to negative cash flows—the cost of covering shortfalls. Don't use it as a universal discount rate or confuse it with your cost of capital, which might apply differently across projects.
  2. Setting unrealistic reinvestment rates — Choosing a reinvestment rate that exceeds your expected cost of capital inflates MIRR. Use market benchmarks or your organisation's long-term capital return expectations rather than the project's IRR.
  3. Ignoring the project timeline — MIRR is sensitive to when cash flows occur. A project with later inflows will show a lower MIRR than an earlier-paying project with identical total cash, because reinvestment compounds for fewer periods.
  4. Overlooking negative terminal cash flows — If your project requires a large salvage or disposal cost in the final year, this acts as a negative flow at the end. Failure to include it understates the true cost of capital recovery.

MIRR vs. IRR: When to Use Which

IRR remains useful for quick screening and when projects have similar risk profiles and reinvestment assumptions. However, MIRR is superior for:

  • Capital-constrained environments: Where reinvestment at the project's IRR is implausible
  • Mixed cash flow patterns: Projects with multiple sign changes benefit from explicit rate assumptions
  • Comparing dissimilar projects: MIRR allows apples-to-apples comparison when funding and reinvestment contexts differ

In practice, calculate both metrics. If MIRR and IRR diverge significantly, investigate whether your financing and reinvestment rate assumptions align with reality. The gap often reveals hidden assumptions baked into your cost of capital estimates.

Frequently Asked Questions

What is the difference between MIRR and IRR?

IRR assumes all intermediate cash inflows are reinvested at the IRR itself, which is rarely realistic for most projects. MIRR corrects this by allowing you to specify separate financing and reinvestment rates tied to your actual capital market conditions. This typically produces a lower, more conservative return estimate than IRR, especially for projects with lumpy or irregular cash flows spanning multiple years.

What should I use as the financing rate in MIRR?

The financing rate represents your actual or marginal cost of debt for negative cash flows. Use your company's cost of borrowing (secured or unsecured loan rate), weighted average cost of capital (WACC), or cost of equity if the project is equity-financed. For projects covering shortfalls from operations, use your internal cost of capital. Avoid using the project's expected return—use market-observable or internal cost metrics instead.

How do I choose a realistic reinvestment rate?

The reinvestment rate should reflect what you can actually earn by deploying excess cash generated by the project. Common choices include your cost of capital, average market return on safe investments (e.g., short-term bonds), or your organisation's required rate of return. Be conservative: overstating this rate inflates MIRR and can lead to poor investment decisions. If uncertain, use your cost of capital as a baseline.

Can MIRR be negative?

Yes. A negative MIRR means the present value of negative cash flows (discounted at the financing rate) exceeds the future value of positive flows (compounded at the reinvestment rate). This signals a value-destroying project that fails to cover its cost of capital. Even if the project breaks even in accounting terms, a negative MIRR indicates it should be rejected in favour of alternative uses of the capital.

Why does MIRR give a different result than NPV?

MIRR and NPV answer different questions. NPV measures absolute wealth creation at a specific discount rate (your cost of capital); MIRR is the discount rate that makes NPV zero under your specified financing and reinvestment assumptions. A project with positive NPV at your cost of capital typically has an MIRR exceeding that rate, but not always—especially if cash flow timing is unusual. Both metrics should drive the same accept/reject decision if your assumptions are consistent.

Does MIRR handle irregular cash flows well?

MIRR handles irregular cash flows better than IRR because you control the reinvestment and financing assumptions directly. However, like all return metrics, MIRR can still be ambiguous if there are multiple sign changes in cash flows (e.g., initial investment, then inflows, then a large cleanup cost at the end). In such cases, run sensitivity analysis on your rate assumptions to ensure the MIRR ranking is robust.

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