How this instrument works
Weighted average cost of capital blends what a company's shareholders require with what its lenders charge, in proportion to how much of the firm is actually financed by each source. Equity and debt are weighted by market value, not the figures sitting on a balance sheet — book equity is what a firm paid for its assets minus depreciation, while market value of equity is what the stock trades for today, and it shifts every time the share price moves. Debt often sits close to book value since loans trade near par, but for bonds priced well above or below face value, the market price belongs in this formula, not the amount originally borrowed.
The debt leg is multiplied by (1 − tax rate) and the equity leg is not, because interest lowers a company's tax bill and dividends do not — the same tax shield priced separately on an after-tax cost of debt sheet, baked directly into this blend instead. A CFO builds this number once, at the whole-company level, then uses it as the discount rate for a discounted cash flow valuation or as the hurdle rate a proposed project's expected return has to clear before capital gets committed to it. A private-equity analyst pricing an acquisition target runs the same blend using the target's own market-implied weights, not the acquirer's.
WACC describes the average risk of everything a company already does, so applying it unchanged to a project riskier or safer than that average systematically misprices the decision — a mining company evaluating a low-risk logistics investment should discount those cash flows at a lower rate than its mining WACC implies, or it will reject perfectly good projects. The blend also assumes the entered weights hold going forward; a business planning to raise significantly more debt or equity than its current mix should recompute WACC at the target structure, not the one sitting on today's balance sheet.
- Enter the company's Market value of equity, $ — share price times shares outstanding, not the book value on the balance sheet.
- Enter Market value of debt, $ — the market price of outstanding bonds and loans, or their face value if none trades separately.
- Set Cost of equity, % to the return shareholders require, often taken from a CAPM or dividend-growth calculation.
- Set Cost of debt (pre-tax), % to the rate lenders actually charge, and Tax rate, % to the marginal rate that shields the interest.
- Read WACC, % — the single blended rate to carry into a discounted cash flow valuation or a capital-budgeting decision.
Worked example — 70% equity, 30% debt at 9.75%
Take the golden case: $700,000 of equity costing 12%, $300,000 of debt costing 6% pre-tax, and a 25% tax rate. Total capital V = $1,000,000, so equity is 70% of the mix and debt is 30%. The equity leg contributes 0.70 × 12% = 8.4 percentage points. The debt leg is taxed first — 6% × (1 − 0.25) = 4.5% after-tax — then weighted at 0.30 × 4.5% = 1.35 percentage points. Add the two legs and WACC = 8.4% + 1.35% = 9.75%, the rate this company would use to discount its own future cash flows.
Hold the individual rates fixed and change only the mix to see why capital structure matters: an all-equity version of the same company, with zero debt, has WACC exactly equal to its 12% cost of equity, since the debt term has nothing left to weight. Move to a 50/50 split instead and WACC falls to 8.25%, because debt is the cheaper source once its tax shield is counted — which is also why more leverage does not lower WACC forever. Heavier borrowing eventually raises the risk lenders and shareholders both perceive, pushing Re and Rd back up in turn.
Questions
Why use market value instead of book value for the weights?
Because WACC prices what investors would have to be paid today to hold the company's equity and debt, and that price is set by the market, not by historical accounting entries. Using book value of equity — original cost minus depreciation — instead of what the stock actually trades for is one of the most common WACC mistakes, and it can shift the answer by several percentage points for a company whose share price has moved a lot since issue.
Why does the tax rate only apply to the debt side of the formula?
Interest paid to lenders is deductible before a company's tax bill is calculated, so every dollar of interest genuinely costs less than a dollar; dividends and buybacks paid to shareholders come from profit that has already been taxed, so equity gets no equivalent discount. Multiplying only the debt leg by (1 − tax rate) reflects that real asymmetry, not an adjustment applied for symmetry's sake.
Who actually calculates WACC, and for what decision?
Corporate finance teams compute it to discount a company's projected free cash flows in a valuation, and to set the hurdle rate a proposed project's expected return must beat before capital is committed. Private-equity and investment-banking analysts run the same blend, using a target company's own market-implied weights, when pricing an acquisition or building a leveraged buyout model.
Should every project inside a company be discounted at the same WACC?
No — WACC reflects the average risk of the company's existing business, and a project meaningfully riskier or safer than that average deserves its own adjusted discount rate. Using one company-wide WACC for every decision systematically favors riskier projects, which look artificially attractive when discounted at a rate too low for the risk they actually carry.
Does taking on more debt always lower WACC?
Only up to a point. Debt is usually cheaper than equity, especially after its tax shield, so shifting the mix toward debt lowers the blended rate at first — moving this example from 70/30 to a 50/50 split drops WACC from 9.75% to 8.25%. Push leverage further, though, and rising bankruptcy risk pushes both the cost of debt and the cost of equity up, eventually outweighing the extra debt weight.
How does this relate to the separate cost of equity and cost of debt figures?
This instrument does not derive Cost of equity or Cost of debt itself — it takes both as inputs and blends them by market-value weight. Cost of equity is typically produced by a CAPM or dividend-growth calculation, and pre-tax cost of debt by a lender's quoted rate; this sheet's only job is the weighting and tax adjustment that turns those two separate numbers into one discount rate.
References
- NYU Stern (Damodaran) — cost of capital and WACC estimation resources
- U.S. SEC Investor.gov — investing basics, risk, and glossary
Read this first: This instrument shows arithmetic, not advice. Real offers add fees, taxes and terms that vary by lender and place — verify the figures against your actual paperwork before deciding anything.