NPV vs IRR: Which Rule Should Rule Your Capital Budget?

Every organisation that deploys capital eventually confronts the same fork: two seemingly equivalent profitability measures that disagree. Net present value (NPV) computes the wealth a project adds in currency terms; internal rate of return (IRR) computes the percentage yield the project embeds. On simple, well-behaved projects they point the same way. But real portfolios are full of projects of different sizes, different lives, and awkward cash-flow shapes — and it is precisely on those projects that the two rules split, forcing a choice that many analysts make by habit rather than understanding.

Both rules, one idea

Start from the common ground. Both rules discount every cash flow to the present using the opportunity cost of capital. NPV then sums them, subtracting the initial investment, and reports a currency surplus: NPV = Σ CF_t / (1 + r)^t − I₀. A positive NPV means that after repaying capital with its required return, the project still leaves extra wealth on the table. IRR instead asks what single discount rate r makes that surplus exactly zero — the yield at which the project merely breaks even in present-value terms. Accept if IRR exceeds the hurdle rate; reject if it falls below.

On independent, conventional projects — one outflow followed by inflows, no capital rationing — the two rules always agree. The trouble begins the moment real life adds asymmetries: scale differences, timing differences, non-conventional cash-flow signs, or mutually exclusive choices. Each of these breaks the agreement in a characteristic way.

Conflict one: scale

Project A invests 10,000 and returns 12,000 next year — an IRR of 20% and, at a 10% discount rate, an NPV of about 909. Project B invests 1,000,000 and returns 1,080,000 next year — an IRR of 8% but an NPV of about −18,182 at 10%... or flip the example: B earns 1,150,000, giving IRR 15% and NPV 45,455. B's IRR is lower, yet B makes you 50 times wealthier than A. Percentage return measures efficiency per unit of capital; NPV measures absolute wealth created. When the projects are mutually exclusive and capital is available, wealth is the objective — and only NPV measures it directly.

Conflict two: timing and the reinvestment assumption

IRR implicitly assumes every intermediate cash flow is reinvested at the IRR itself. A project earning 30% is assumed to sweep its year-one receipts into new 30% opportunities until the horizon ends. NPV assumes intermediate cash is reinvested at the discount rate — roughly, at the return of comparable market opportunities. The NPV assumption is modest and defensible; the IRR assumption is flattering and frequently false. Between two projects with the same total cash but different timing, IRR systematically favours the one that pays back early, whether or not early cash truly can be redeployed at those rates.

Conflict three: cash flows that change sign

Whenever cash flows swing from negative to positive and back — projects with decommissioning costs, mines with restoration obligations, funds with interim distributions — the IRR equation can have multiple mathematical roots or none. A project can display two legitimate IRRs (say 6% and 42%) with no way to prefer one, or no IRR at all even though its NPV is comfortably positive at every plausible rate. NPV, by contrast, is always well-defined: one discount rate in, one currency value out. Our IRR guide works through a dual-IRR example in full, and the NPV profile visual tool draws the curve so you can see the crossings with your own eyes.

Conflict four: mutually exclusive projects with different lives

A five-year machine and a ten-year machine are not directly comparable on either raw NPV or IRR: the ten-year option delivers twice the service period. The fair comparison uses equivalent annual cost or annuity methods — restating each project's NPV as an equal annual amount over its life. Once restated, NPV logic applies cleanly and IRR adds nothing, because the annualisation step already embedded the time value. This is the scenario where consulting firm surveys show the highest rate of rule misuse.

IRR tells you the shape of the return; NPV tells you its size. Strategy lives on size — you cannot pay dividends with percentages.

How professionals actually decide

• Screen with simple metrics: payback and profitability index triage a long list quickly and communicate intuitively.
• Decide with NPV: for ranking and selection under normal conditions, NPV is the criterion that aligns with shareholder wealth.
• Report IRR alongside for communication: boards understand yields; just discipline the comparison to same-scale, same-life projects.
• Use MIRR when the reinvestment objection matters: modified IRR lets you specify a realistic reinvestment rate explicitly.
• Watch the discount rate: both rules are only as good as the cost of capital plugged into them.