Marginal efficiency of capital is the expected discount rate that equates a new capital asset's prospective yields with its supply price.
The marginal efficiency of capital (MEC) is the discount rate that makes the present value of a new capital asset’s expected net yields equal to its current supply price. In Keynesian investment theory, it represents the expected rate of return over cost on an additional capital asset, based on future expectations rather than the historical performance of an existing asset.
MEC helps describe investment demand: businesses have more incentive to undertake projects whose expected MEC exceeds the relevant financing or opportunity-cost threshold. This is a theoretical framework, not a rule that every project with an estimated rate above a bank loan rate should proceed.
If a capital asset has supply price P_s and expected net yields Q_t over n periods, MEC is the rate m satisfying:
The expected yields should be incremental amounts attributable to the asset after relevant operating costs. The model is expectation-based; changing expected demand, prices, costs, life, or residual value changes the estimated MEC.
Suppose a machine has a supply and installation price of 100,000 and is expected to generate net yields of 60,000 at the end of each of two years, with no residual value. MEC solves:
The solution is approximately 13.1%. In a simplified model, the project appears attractive if the relevant threshold is 9% and unattractive if it is 15%.
That comparison is not a complete investment appraisal. The analyst still needs to test whether the cash-flow forecast incorporates risk, taxes, working capital, downtime, financing interactions, alternative projects, and the possibility of delaying the decision.
MEC is a marginal concept. Firms rank prospective additions to capital by expected return. As more investment is contemplated:
The resulting schedule relates the quantity of proposed investment to the expected marginal rate. A lower interest-rate or opportunity-cost threshold can make more projects appear viable, but weak demand expectations can lower the MEC schedule at the same time.
| Measure | Core meaning | Important distinction |
|---|---|---|
| Marginal efficiency of capital | Expected discount rate implied by new asset yields and supply price | Keynesian investment-demand concept |
| Internal rate of return | Discount rate that sets a specified project’s NPV to zero | General project-appraisal calculation |
| Marginal product of capital | Additional physical output from additional capital | Output concept, not directly a cash return |
| User cost of capital | Period cost of employing a capital asset | Includes financing opportunity cost, depreciation, and expected price change |
| Cost of capital | Required financial return for providers of funds | Depends on financing and risk assumptions |
MEC and IRR can use the same present-value mathematics. The term MEC emphasizes expectations for newly produced capital assets and the aggregate inducement to invest; IRR is the more common modern project-finance label.
Some educational sources use marginal efficiency of investment (MEI) as an alternate label for the expected project-return or investment-demand schedule. In that usage, a separate generic MEI calculation adds little beyond MEC and IRR.
Modern macroeconomic models can use “marginal efficiency of investment shock” differently. There it may describe a change in how efficiently investment goods become installed capital, not the discount rate that equates expected project cash flows with cost. Readers should identify the model before treating MEI as a synonym.
MEC connects expectations, asset prices, interest rates, and investment spending. It helps explain why investment can fall even when borrowing rates decline: expected future yields may deteriorate by more than financing conditions improve.
For businesses, the concept reinforces the need to forecast incremental economics rather than extrapolate historical accounting returns. For macro analysis, it highlights how confidence and expected demand can shift the investment schedule.