Every capital decision is a comparison against the alternatives, including doing nothing. These calculators put a defensible number on that comparison rather than relying on the instinct that a project "feels worth it".
Which one you want
ROI is the simple, timing-blind version — useful for quick comparisons, misleading over long horizons because it treats a dollar in five years as a dollar today. NPV and IRR correct that by discounting future cash flows, and MIRR fixes IRR's optimistic assumption that interim cash is reinvested at the IRR itself. WACC produces the discount rate those models need — the blended cost of the debt and equity funding the business. Payback Period answers the cash-risk question rather than the return question: how long until the money comes back. Rule of 72 is the mental shortcut for how long a rate takes to double a sum.
What the numbers mean
NPV and IRR can disagree, and when they do, NPV is the one to trust for mutually exclusive projects — it measures value created rather than a rate. Both are only as good as the cash-flow forecast underneath, which is where the real uncertainty sits. Run each model at a pessimistic, expected and optimistic case; a project that only clears the hurdle in the optimistic case is a bet, not an investment.