ROI / NPV / IRR Calculator

The CFO asks whether a $200k equipment purchase is worth it. You can answer three different ways, and each tells a different story. ROI gives the simplest ratio — total gain divided by total cost — but ignores when the money arrives. NPV fixes that: it discounts every future cash flow back to today at a chosen discount rate, then subtracts the upfront cost. A positive NPV means the investment creates value after accounting for the time value of money. IRR flips the question: at what discount rate does NPV equal zero? The common mistake is using ROI alone for multi-year projects — a 40% ROI spread over eight years looks very different from 40% in one year.

Use ROI for quick back-of-napkin comparisons. Use NPV when you need a dollar figure for how much value the project adds. Use IRR when you want a rate you can compare against your cost of capital or a competing project’s IRR. All three assume your cash-flow estimates are correct — garbage in, garbage out.

NPV is extremely sensitive to the discount rate. At 8%, a $50k annual cash flow for five years is worth $199k today. At 15%, the same stream is worth $168k. A seven-point change in the rate swings NPV by $31k — enough to flip a borderline project from “go” to “no-go.” If you are unsure which rate to use, run the analysis at your company’s weighted average cost of capital (WACC) as the base case, then stress-test at WACC ± 3 points.

Cash-flow timing matters almost as much. Front-loaded projects (big returns in years 1–2) have higher NPV than back-loaded ones (returns in years 4–5) at the same total. IRR amplifies this: it rewards early cash flows heavily because reinvestment happens sooner. If your project has lumpy, uneven cash flows, IRR can produce misleading results — sometimes even multiple IRRs when cash flows change sign more than once. In those cases, MIRR (modified IRR) is more reliable.

Each metric answers a different question. Here is when to reach for which:

Comparing IRRs across projects of different sizes. Project A has a $10k investment and 50% IRR ($5k gain). Project B has a $500k investment and 20% IRR ($100k gain). IRR says pick A, but NPV says B creates twenty times more value. IRR ranks efficiency; NPV ranks magnitude. For capital allocation, you usually care about magnitude.

Using ROI without a time dimension. “ROI is 120%” means nothing without knowing the period. 120% over one year is stellar; 120% over ten years is about 8% annualised — barely beating a savings account. Always annualise ROI or state the holding period explicitly.

Ignoring opportunity cost. A project with a positive NPV at 10% discount rate is still a bad investment if you have an alternative project with a higher NPV using the same capital. NPV tells you whether a project clears the hurdle, not whether it is the best use of limited funds. Compare NPVs across the set of mutually exclusive options.

Non-conventional cash flows. If a project requires a large decommissioning cost in the final year (positive flows, then negative), the cash-flow series changes sign twice. The IRR equation can produce two valid solutions — say 8% and 35%. Neither is “the” IRR. Use MIRR instead, which assumes a separate reinvestment rate and finance rate, giving one clean answer.

NPV exactly zero. This means the project earns exactly the discount rate — no more, no less. It does not mean the project is worthless; it means you are indifferent between doing it and investing at the discount rate. In practice, a zero-NPV project might still be worth doing if it has strategic value the model does not capture.

Very long horizons. A 20-year NPV is heavily dependent on cash-flow estimates for years 10–20, which are usually guesses. Discount rates compress distant cash flows, but if the rate is low (3–5%), those distant years still carry significant weight. Sensitivity-test the terminal value or cap the projection at a conservative horizon.

The core formulas behind each investment metric:

Scenario: A manufacturing firm considers buying a $150k CNC machine. Expected cash flows: Year 1 = $40k, Year 2 = $50k, Year 3 = $55k, Year 4 = $45k, Year 5 = $30k. The firm’s WACC is 10%.

ROI: Total gains = $220k. ROI = ($220k − $150k) / $150k = 46.7% over five years. Annualised ≈ 8.0%. Useful as a headline but does not account for timing.

NPV at 10%: $40k/1.10 + $50k/1.21 + $55k/1.331 + $45k/1.4641 + $30k/1.6105 − $150k = $36.4k + $41.3k + $41.3k + $30.7k + $18.6k − $150k = +$18.3k. Positive NPV — the project clears the hurdle and creates $18.3k in present-value terms.

IRR: Solving iteratively, NPV hits zero at r ≈ 16.2%. Since 16.2% > 10% WACC, the project earns more than the cost of capital. Payback: Cumulative cash flow turns positive partway through Year 3 ($40k + $50k + $55k = $145k at end of Y3, full payback early in Y4 after $5k more flows in).