WACC tax shield: why only debt gets the tax break

Corporate Issuers·July 30, 2026·4 min read

A valuation question gives a company’s debt and equity mix, then asks for its weighted average cost of capital (WACC). A candidate who weights the quoted costs without adjusting debt reaches 10.40%, because the pre-tax debt cost has been treated as the final input. The arithmetic looks tidy, and every number came from the question. The mistake sits in one missing factor.

Only debt carries the tax shield

WACC combines the required return for each source of financing in proportion to its share of total capital. Equity and debt enter the same weighted average, but their tax treatment is not symmetrical.

WACC = (E ÷ V) × r_e + (D ÷ V) × r_d × (1 − t_c)

The factor (1 − t_c) multiplies the cost of debt because interest is tax deductible. Corporate taxes reduce the effective cost that the company bears. Dividends paid to common shareholders are not deductible, so the equity branch receives no corresponding reduction. That difference is the tax shield, the source of debt’s value in MM proposition I with taxes.

The adjustment belongs to the issuer’s financing cost, not to every return inside the discount rate. Shareholders still require r_e, with no corporate tax discount attached to it. The debt branch therefore contains a weight, a required return, and a tax factor, while the equity branch stops after the weight and required return. What looks like a small algebraic difference reflects how interest and shareholder distributions pass through corporate taxes.

The weights come from capital policy

Correct tax treatment cannot rescue the wrong weights. When a question supplies a target capital structure, those target proportions take priority because WACC is intended to reflect the financing mix the company expects to maintain, not an accidental snapshot from its accounting records.

If no target is supplied, the structure has to be estimated. Current market value weights are the usual starting point, while the company’s own capital structure trend or the average for a comparable group can also support the estimate. Book values do not belong in the calculation. They record historical accounting amounts and can sit far from the economic values on which investors base their required returns.

Worked example: two familiar wrong answers

Suppose E ÷ V = 0.6, D ÷ V = 0.4, r_e = 12%, r_d = 8%, and t_c = 25%. The debt cost is multiplied by 0.75, while the equity cost remains at 12%.

WACC = 0.6 × 12% + 0.4 × 8% × 0.75

WACC = 7.20% + 2.40% = 9.60%

The correct WACC is 9.60%. Omitting (1 − t_c) produces 10.40%, the high distractor from treating debt as if interest offered no tax deduction. Applying the factor to both financing branches produces 7.80%, the low distractor from treating common dividends like deductible interest.

Quick reference

A company with preferred stock exposes a limitation that the two-branch formula does not display. Preferred stock adds another branch to WACC, yet its cost receives no tax reduction because preferred dividends are not deductible. Grouping preferred stock with debt merely because both may pay a stated rate would import the debt tax shield into the wrong source of capital, even if every weight and required return were otherwise correct.

The same economic reading matters in the cash conversion cycle sign trap: in both cases, a component’s role determines whether it reduces the final measure.

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