Key Assumptions Behind the Net Present Value Method
Every financial model rests on assumptions. Understand the implicit reinvestment rate and liquidity assumptions behind NPV calculations.
Key Takeaways
- Reinvestment Assumption: Intermediate cash flows are reinvested at the discount rate (WACC), which is more realistic than IRR’s reinvestment assumption.
- Certainty Equivalence: Cash flows are assumed known or properly probability-weighted.
- Linear Discounting: The hurdle rate is assumed constant across time unless explicit multi-period rates are modeled.
1. The Reinvestment Rate Assumption
A major theoretical strength of NPV over IRR is its reinvestment assumption. NPV assumes all intermediate cash inflows can be reinvested at the company’s cost of capital ($r$), whereas IRR assumes they can be reinvested at the internal rate of return itself (which may be unrealistically high, like 50% or 100%).
2. Fixed Discount Rate and Market Efficiency Assumptions
Standard NPV models assume a single, constant discount rate across the entire multi-year forecast horizon. In volatile interest rate regimes, sophisticated financial teams implement term-structure discounting (different hurdle rates for Years 1–3 vs Years 4–10) to reflect macroeconomic yield curve realities.
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