Profitability Index Explainer
The profitability index compares the present value of a project’s expected cash inflows with the present value of its cash outflows. A common formula divides discounted benefits by discounted costs. A result above one corresponds to a positive net present value under the same assumptions.
Because it is a ratio, the index can help rank projects when capital is limited. A project promising more present value per dollar of constrained investment may receive priority. The calculation must use consistent timing, discount rates, taxes, working capital, and terminal cash flows.
Ranking by ratio can conflict with total value. A small project may have a higher index but add fewer dollars of NPV than a larger project. Projects can also be indivisible, mutually exclusive, dependent on one another, or limited by staff and capacity rather than money alone.
The index inherits every weakness in the cash-flow forecast and discount rate. It should support, not replace, NPV and strategic judgment. When constraints exist across several periods, formal optimization and scenario analysis may be more useful than a simple one-time ranking.
Divide present value of inflows by present value of outflows.
A value above one aligns with positive NPV using the same inputs.
The ratio shows estimated value relative to invested dollars.
It can help screen projects when funding is constrained.
The highest ratio need not create the most total value.
Use NPV and consider project size, dependencies, and constraints.
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