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The One-WACC-Fits-All Fallacy

The WACC fallacy is applying one firm WACC to every project regardless of risk. It is common. Survey evidence shows that more than half of firms use a single companywide rate. The fallacy systematically misallocates capital, because the single rate over-prices safe projects and under-prices risky ones relative to their true cost of capital. The result is a tilt toward risky investments whose IRR happens to beat the firm rate, and the rejection of safe projects that would add value at a correctly lower rate.

Why it matters

It is tempting to compute one WACC and stamp it on everything, and for a single-business firm that is often fine. The error appears once projects differ in risk. The single rate is too low for risky projects, so they clear too easily, and too high for safe projects, so they fail when they should pass. Over many decisions the firm loads up on risk and starves its safer, value-adding opportunities. There are offsetting benefits, simplicity, lower administrative cost and a sense of fairness across project sponsors, which is why so many firms accept the trade-off knowingly.

Formulas

The fallacy stated
rproject=WACCfirm∀ projects, regardless of βprojectr_{project} = WACC_{firm} \quad \forall\ \text{projects, regardless of } \beta_{project}
Applying the firm WACC to every project ignores differences in βproject\beta_{project}. It is correct only for projects whose risk matches the firm average, and biased for all others.
The direction of the error
βproject>βfirm⇒WACCfirm<rprojecttrue\beta_{project} > \beta_{firm} \Rightarrow WACC_{firm} < r_{project}^{true}
For a riskier-than-average project the firm rate sits below the true required return, so the project looks better than it is. The reverse holds for a safer-than-average project.

Worked examples

Scenario

A firm with a single WACC of 10 percent runs one safe division and one risky division. Over several years it keeps approving risky-division projects with IRRs just above 10 percent and rejecting safe-division projects with IRRs just below 10 percent. What happens to the firm, and what would survey evidence predict about how common this is?

Solution

The risky projects clear the 10 percent bar because the single rate understates their true required return, while the safe projects fail it because the single rate overstates theirs. Year after year the firm accumulates risky projects and forgoes safe value-adding ones, so its overall risk rises and capital is misallocated. Survey evidence is that more than half of firms use a single companywide rate, so this fallacy is widespread, not a corner case.

Common mistakes

  • ✗A single firm WACC is simply wrong and should never be used. For a firm with a narrow, uniform range of activities it is a reasonable approximation. The fallacy is applying it across genuinely different risks.
  • ✗The single-rate bias goes in both directions equally and cancels out. It does not cancel. It consistently favours risky projects and rejects safe ones, so the firm drifts riskier over time.
  • ✗Using one rate has no benefits at all. It is simpler, cheaper to administer and seen as even-handed across sponsors. These are real benefits weighed against the bias.
  • ✗The fallacy is rare in practice. Survey evidence shows most firms default to a single rate, so it is common, not exceptional.

Revision bullets

  • •The fallacy applies one firm WACC to every project regardless of risk
  • •Survey evidence: more than half of firms use a single companywide rate
  • •The single rate over-prices safe projects and under-prices risky ones
  • •The firm tilts toward risky projects and rejects value-adding safe ones
  • •Capital is misallocated and overall firm risk rises over time
  • •Benefits of one rate: simplicity, lower cost, perceived fairness

Quick check

The main problem with applying one firm-wide WACC to every project is that it

Despite its bias, many firms still use a single discount rate because it is

Connected topics

WACCProject vs Firm RateDivisional WACCHurdle RatesProject Risk Rate

Sources

  1. Titman & Martin, Ch. 5
    Titman, S. & Martin, J. D. Valuation: The Art and Science of Corporate Investment Decisions. Pearson.
    Chapter on the costs and benefits of multiple versus single discount rates. The capital-misallocation argument follows this text.
  2. Graham & Harvey (2001)
    Graham, J. R. & Harvey, C. R. "The Theory and Practice of Corporate Finance: Evidence from the Field." Journal of Financial Economics, 60(2-3), 2001, pp. 187-243.
    Survey evidence that more than half of firms use a single companywide discount rate for all investment proposals.
How to cite this page
Dr. Phil's Quant Lab. (2026). The One-WACC-Fits-All Fallacy. Derivatives Atlas. https://phucnguyenvan.com/concept/sabv-wacc-fallacy
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