The cost of capital is the single most consequential number in corporate valuation. Shift it by one percentage point and an entire company appears dramatically undervalued or dangerously overpriced. WACC — the weighted average cost of capital — is how analysts calculate it, and understanding its mechanics is fundamental to serious financial modelling.
The formula and what each component means
WACC is calculated as:
Where E is market value of equity, D is market value of debt, V = E + D (total firm value), Re is cost of equity, Rd is pre-tax cost of debt, and T is the corporate tax rate.
Cost of equity: the number you cannot observe
Unlike the cost of debt — which is observable in market yields — cost of equity has no contract attached to it. Shareholders do not receive a fixed, promised return. Analysts estimate it using the Capital Asset Pricing Model (CAPM):
Re = Rf + β × (Rm − Rf)
The risk-free rate (Rf) is typically the yield on a 10-year government bond in the company's home currency. Beta (β) measures the stock's sensitivity to market movements — a beta of 1.5 means the stock moves 1.5% for every 1% market move. The equity risk premium (Rm − Rf) is the excess return investors historically demand for holding equities over risk-free assets, typically 4–6% in developed markets.
Cost of debt and the tax shield
Cost of debt should reflect the company's marginal cost of borrowing — the rate it would pay on new debt today — not the average historical rate. For public companies this is observable from the yield-to-maturity on traded bonds. For private companies, analysts benchmark against comparable rated debt or use the rate on the most recent credit facility.
The critical adjustment is the tax shield: because interest payments are tax-deductible in most jurisdictions, the effective after-tax cost of debt is Rd × (1−T). A company paying 6% interest with a 25% tax rate has an after-tax cost of debt of only 4.5% — the government subsidises the remaining 1.5%.
How WACC is used in practice
WACC is the discount rate applied to free cash flows in a DCF model. Each projected cash flow is divided by (1 + WACC)^n to express its present value — reflecting that a pound received in year 5 is worth less than a pound today by the opportunity cost of capital. WACC is also used internally as a hurdle rate: the minimum return on investment a project must generate to create value. Investments returning below WACC destroy value even if they are nominally profitable.
The most common modelling mistakes
Using book value weights instead of market value weights is the most frequent error. A company with minimal equity book value but a high market cap produces a very different WACC using the wrong weights. Using the coupon rate on existing bonds rather than the current yield-to-maturity is another — if rates have risen since issuance, the coupon understates the true cost of debt significantly.
A subtler issue arises in LBO modelling: debt is repaid over time, changing the capital structure — and hence WACC — in every projected year. Serious models recalculate WACC annually, or use the adjusted present value (APV) method, which separates the unlevered business value from the tax shield and avoids circular dependency between the discount rate and the debt balance.
Why a one-point change in WACC changes everything
Because terminal value — the present value of all cash flows beyond the explicit forecast period — typically accounts for 60–80% of a company's enterprise value in a DCF, and because terminal value is computed as FCF ÷ (WACC − g), a 1% increase in WACC produces a disproportionately large decline in valuation. For a mature company with a 2% terminal growth assumption, moving WACC from 9% to 10% reduces terminal value by roughly 14%. This is why analysts always present DCF outputs as valuation ranges across a WACC sensitivity table rather than point estimates — the terminal value assumption swamps everything else.