The profitability index (PI) is useful when a business must compare the value created per unit of investment. However, PI is a ranking tool—not a complete decision system. It can produce a misleading answer when projects differ in size, are indivisible, are mutually exclusive, or compete for funds across several periods.
What is the profitability index?
Profitability index compares the present value of expected future cash inflows with the initial investment:
PI = Present value of future cash inflows ÷ Initial investment
For a conventional project, PI can also be expressed as:
PI = 1 + (NPV ÷ Initial investment)
- PI > 1: the project creates value at the required return.
- PI = 1: the project is expected to break even in present-value terms.
- PI < 1: the project destroys value.
For the underlying method, see how the profitability index is calculated and compare it with the net present value method.
Simple profitability index example
| Project | Investment | PV of inflows | PI | NPV |
|---|---|---|---|---|
| A | $100,000 | $125,000 | 1.25 | $25,000 |
| B | $40,000 | $52,000 | 1.30 | $12,000 |
Project B has the higher PI, but Project A produces the higher NPV. If the projects are mutually exclusive and the company is not capital-rationed, choosing B only because its PI is higher would sacrifice $13,000 of value.
Main limitations of profitability index
1. PI can ignore the scale of investment
PI is a relative measure. A small project can generate an excellent ratio while adding less total value than a larger project. This is the main reason PI and NPV may give conflicting rankings for mutually exclusive projects.
2. It can fail with indivisible projects
Real projects cannot always be divided into smaller portions. Suppose a firm has a $100,000 budget. Project X requires $60,000, has a PI of 1.50 and creates an NPV of $30,000. Project Y requires the full $100,000, has a PI of 1.40 and creates an NPV of $40,000. A simple PI ranking starts with X, but the remaining $40,000 cannot be invested. Choosing Y creates $10,000 more value.
3. A simple PI ranking may not find the best project combination
Under capital rationing, managers often rank projects by PI and accept them until the budget is exhausted. That shortcut can fail when project sizes do not fit the available budget. The correct solution may require evaluating combinations, using integer programming, or applying another optimisation method.
Read the related lesson on investment decisions under capital rationing.
4. Multi-period capital constraints create a timing problem
A project selected today can change the cash available for projects next year. A high-PI project may consume funds now but generate insufficient early cash flows, preventing the company from accepting a valuable future opportunity. A single-period PI ranking does not capture this sequence well.
5. PI is unreliable for mutually exclusive or dependent projects
Mutually exclusive projects require one choice among alternatives, while dependent projects must be considered together. A stand-alone PI for each project may ignore these relationships. Incremental cash-flow analysis and NPV are normally more appropriate.
6. Results depend on forecast quality and the discount rate
PI is only as reliable as its inputs. Overstated cash flows, ignored risks, an unrealistic project life, or an unsuitable discount rate can make an unattractive project appear acceptable. Scenario and sensitivity analysis should test the assumptions. See risk analysis in capital budgeting.
7. It does not automatically value managerial flexibility
The ability to delay, expand, abandon or change a project can be valuable. A basic PI calculation misses these real options unless the expected cash flows or project valuation explicitly includes them.
Profitability index versus NPV
| Decision situation | Preferred approach |
|---|---|
| Independent project with no capital constraint | Accept if NPV > 0; PI > 1 gives the same accept/reject signal. |
| Mutually exclusive projects | Choose the project with the highest positive incremental NPV. |
| Single-period capital rationing | Use PI as a screening guide, then test feasible project combinations. |
| Multi-period constraints | Use a multi-period optimisation model and strategic review. |
Practical decision checklist
- Estimate incremental after-tax cash flows.
- Select a risk-appropriate discount rate.
- Calculate both PI and NPV.
- Identify mutual exclusivity, dependence and indivisibility.
- Test funding limits for every relevant period.
- Compare feasible project combinations—not only individual rankings.
- Run sensitivity and scenario analysis before approval.
Frequently asked questions
What is the biggest limitation of profitability index?
Its relative ranking can favour a smaller project even when a larger project creates more total NPV.
Can PI and NPV give different rankings?
Yes. Conflicts commonly occur when mutually exclusive projects have different investment sizes or timing patterns.
Is a PI greater than 1 always enough?
No. It is an initial accept signal for an independent project, but managers must still assess risk, strategy, funding constraints and alternative projects.
Why is PI useful under capital rationing?
It measures value created per unit invested, which helps screen projects when capital is scarce. It does not guarantee the best combination when projects are indivisible.
Should a company use PI or NPV?
Use both, but normally give priority to NPV when the objective is maximising total value.
Continue learning: compare the main investment appraisal methods and use the Investment Appraisal learning path.
The capital budgeting procedure described above does not always work. It fails in two situations:
- Multi-period capital constraints
- Project indivisibility
The serious limitation in using the PI rule is caused by the multi-period constraints. In the above post example, there is a budget limit of 50000$ year 1 also and the firm is anticipating an investment opportunity 0 as in low is year 1. Thus, the decision choices today are as follows:
Project M and N have the first and second ranks in terms of PI. They together have highest NPV and also exhaust the budget in year 0; so the firm would choose them. Further, projects M and N together are expected to generate 20000$ cash flow next year. This amount with the next year’s budget (i.e. 20000$ + 50000$ = 70000$) is not sufficient to accept project O. Thus, by accepting M and N, the firm will obtain a total NPV of 15870$. However, a careful examination of the project’s cash flows reveals that if project L is accepted now it is expected to generate a cash flow of 30000$ after a year, which together with the budget of 50000$ is sufficient to undertake project O next year. Projects L and O have lower PI ranks than projects M and N, but they have higher total NPV of 19820$.
Project indivisibility
The PI rule of selecting projects under capital rationing can also fail because of project invisibility. It may be more desirable to accept many lower ranked similar projects than a single large project. The acceptance of a single large project, which may be top-ranked, excludes the possibility of accepting small projects, which may have higher total NPV. Consider the following:
Suppose that the firm has budget ceiling of 10$ million. Following the ranking by PI, the firm would choose A and C. These projects spend 850000$ of the total a budget and have a total NPV of 180000$. The next best project E needs an investment of 200000$, while the firm has only 150000$. If we examine the various combinations of projects satisfying the budget limit, we find the package of C, E and D as the best. They exhaust the entire budget and have a total NPV of 189000$. Thus, the firm can choose two lower ranked, small projects, E and D, in place of the higher ranked, large project, A. This section procedure will become very unwieldy if the firm has chosen the best package of projects from a large number of profitable projects.
- Our discussion has shown that the profitability index can be used to choose projects under simple, one-period, capital constraint situation. It breaks down in the case of multi-period capital constraints. It will also not work when any other constraint is imposed, or when mutually exclusive projects, or dependent projects are being considered.
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