A simple ROI calculation answers "how much did I make relative to what I put in," but it treats a dollar received next month exactly the same as a dollar received in year five, which is a distortion serious enough to make a bad investment look identical to a good one on paper.
Why timing changes everything
Money available today can be invested, earning a return between now and whenever a future cash flow would otherwise arrive, so a dollar received sooner is genuinely worth more than a dollar received later, even before accounting for inflation or risk. This is the time value of money, the foundation both NPV and IRR are built on, and it's exactly what a plain ROI percentage leaves out by only comparing total inflows to total outflows regardless of when each occurred.
What NPV actually calculates
NPV discounts every future cash flow back to its present value using a chosen discount rate (often the business's cost of capital or a required rate of return), then sums those present values and subtracts the initial investment. A positive NPV means the investment is expected to add value above what the discount rate alone would have earned; a negative NPV means it's expected to destroy value even if the raw ROI percentage looks positive. Two projects with identical total cash inflows can have very different NPVs if one delivers most of its returns early and the other delivers them mostly in later years, something a simple ROI number can't distinguish at all. The NPV & IRR Calculator runs both figures from the same cash flow schedule.
What IRR adds, and where it can mislead
IRR is the discount rate at which NPV equals exactly zero, essentially the project's break-even rate of return. It's useful for comparing investment opportunities as a single percentage figure, similar to how you'd compare interest rates on different accounts, but IRR can be misleading when comparing projects of very different sizes or cash flow patterns, since a project with a high IRR on a small investment can create less total value than a project with a lower IRR on a much larger one. Comparing NPV and IRR together, alongside a straightforward payback period for how quickly capital is recovered, gives a fuller picture than relying on any single metric alone.