The IRR rule is a guideline used to evaluate whether a project or investment is worth pursuing: if the internal rate of return (IRR) exceeds your minimum required rate of return, accept the project; if it falls below, reject it. That minimum threshold is commonly called the hurdle rate, and it typically reflects the cost of capital, meaning what it costs you to fund the investment through debt, equity, or both.
How the IRR Rule Works
The internal rate of return is the discount rate that makes the net present value (NPV) of all future cash flows from a project equal to zero. In plain terms, it’s the annualized percentage return a project is expected to generate over its lifetime, accounting for the timing and size of every cash inflow and outflow.
The decision logic is straightforward. Say your company’s cost of capital is 10%. If a proposed project has an IRR of 14%, it clears the bar, and the IRR rule says it’s worth pursuing. If the IRR comes in at 7%, the project would earn less than what it costs to finance, so you’d pass. When comparing several projects that all exceed the hurdle rate, a company might still choose one with a lower IRR if it better fits strategic goals, timing, or scale, but only as long as it stays above the cost of capital.
For context on what “good” IRRs look like in practice: top-quartile global buyout funds have historically generated around 24% IRR over the past decade, according to McKinsey, outperforming both the S&P 500 (15% total shareholder return) and the MSCI World index (13%). More recent buyout vintages have shown roughly 15% IRR on a largely unrealized basis. These benchmarks vary widely by industry, asset class, and risk profile, but they give you a sense of the range investors work within.
A Simple Example
Imagine you’re evaluating a $100,000 investment that will generate $30,000 per year for five years. You plug those cash flows into a spreadsheet (most people use the IRR function in Excel or Google Sheets) and get an IRR of about 15.2%. If your hurdle rate is 12%, the project passes. If your hurdle rate is 18%, it doesn’t.
The math behind that calculation solves for the discount rate where the present value of those five $30,000 payments exactly equals the $100,000 you put in. You don’t need to do this by hand. The practical skill is knowing what the result means and how to compare it to your cost of capital.
Where the IRR Rule Falls Short
The IRR rule is popular because it produces a single, intuitive percentage that’s easy to compare across projects. But it has real limitations that can lead to bad decisions if you rely on it alone.
The Reinvestment Assumption
IRR calculations assume that every cash flow generated during the project gets reinvested at the same rate as the IRR itself. If a project shows a 20% IRR, the math assumes you can reinvest interim cash flows at 20% for the entire life of the project. That’s often unrealistic, especially for high-return projects. A project throwing off cash in year two may not have another 20% opportunity waiting. This tends to overstate the actual profitability of a project, which can lead to capital budgeting mistakes based on overly optimistic estimates.
The Modified Internal Rate of Return (MIRR) addresses this by letting you specify a more realistic reinvestment rate, like your actual cost of capital, for interim cash flows. It generally produces a more conservative and arguably more honest estimate of a project’s return.
Non-Conventional Cash Flows
The IRR rule works cleanly when a project has one upfront cost followed by a series of positive returns, sometimes called conventional cash flow. Problems arise when cash flows alternate between positive and negative more than once over the project’s life. A mining operation, for example, might require a large initial investment, generate profits for several years, then require a major cleanup expenditure at the end.
When cash flows change direction multiple times, the IRR equation can produce more than one solution. If a project yields IRRs of both 5% and 15% and your hurdle rate is 10%, you’re stuck: one IRR says accept, the other says reject. In these situations the IRR rule simply breaks down, and net present value analysis becomes the more reliable tool because NPV always produces a single answer.
Scale and Timing Blindness
IRR doesn’t account for the size of an investment. A $10,000 project returning 25% adds $2,500 in value. A $1,000,000 project returning 14% adds $140,000. The IRR rule would rank the smaller project higher, even though the larger one creates far more wealth in absolute terms. Similarly, IRR doesn’t distinguish between a project that delivers returns quickly and one that locks up capital for decades. Two projects can have identical IRRs but very different implications for your cash position and flexibility.
IRR Rule vs. NPV
Net present value and IRR usually agree on whether to accept or reject a single project. If IRR exceeds the hurdle rate, NPV will be positive, and vice versa. The disagreements show up when you’re ranking mutually exclusive projects (where you can only pick one) or when cash flow patterns are unconventional.
NPV tells you the dollar amount of value a project creates. IRR tells you the percentage return. Most finance professionals treat NPV as the more reliable decision tool, especially for ranking projects, because it directly measures value creation and doesn’t suffer from the reinvestment assumption or the multiple-solution problem. The IRR rule works best as a quick screening tool: does this project clear the bar? For the final decision, pair it with NPV.
When to Use the IRR Rule
The IRR rule is most useful in its simplest application: screening investments with conventional cash flows against a known cost of capital. It’s widely used in corporate capital budgeting, private equity, real estate development, and venture capital because it gives stakeholders a single percentage they can quickly compare across opportunities or against market benchmarks.
It works well when you’re making a yes-or-no decision on a standalone project with a clear upfront investment and a stream of future returns. It becomes less reliable when you’re comparing projects of different sizes, dealing with cash flows that flip between positive and negative, or assuming the IRR percentage is the exact return you’ll actually earn. In those cases, supplement it with NPV, MIRR, or payback period analysis to get the full picture.