Cost of Capital (WACC) Calculator
From the weights, cost of equity, cost of debt and tax rate, compute the weighted average cost of capital (WACC).
輸入資料
計算結果
重點速覽：WACC = (E/V) x Re + (D/V) x Rd x (1 - tax rate). E/D = equity/debt market weights, V = E + D, Re = cost of equity, Rd = pretax cost of debt. The (1 - tax) reflects the debt interest tax shield (interest deductible). Example: 70% equity at 12%, 30% debt at 6% pretax, tax 16.5% → ≈9.9%. WACC is the DCF discount rate and investment hurdle. WARNING: More debt doesn't always lower WACC — high leverage raises Re and Rd; the minimum sits at the optimal capital structure, not max debt. Education, not advice.
計算公式
資本成本 = 權益權重 × 權益成本 + 債務權重 × 債務成本 ×(1 − 稅率)。
$$\text{WACC} = w_e R_e + w_d R_d (1 - t)$$使用說明
- Enter the equity and debt weights (should sum to 100%).
- Enter the cost of equity and pretax cost of debt.
- Enter the tax rate to view the WACC.
以權益成本 12%、稅前債務成本 6%、稅率 16.5% 為例，不同債務權重下的資本成本 (WACC)。
| 債務權重 | 權益權重 | 資本成本 (WACC) |
|---|---|---|
| 0% | 100% | 12.00% |
| 20% | 80% | 10.60% |
| 40% | 60% | 9.20% |
| 60% | 40% | 7.81% |
| 80% | 20% | 6.41% |
理財情境案例
案例一：利息稅盾如何拉低債務的實際成本
某香港公司向銀行借款，稅前利率 (債務成本) 為 6%，適用利得稅率 16.5%。
由於利息支出可在計稅前扣除，公司每付出 6% 利息，就能少繳約 6% × 16.5% ≈ 1% 的稅，因此債務的「稅後實際成本」= 6% ×(1 − 0.165) = 5.01%。
這正是為何 WACC 公式要對債務成本乘 (1 − 稅率)：借貸的名目利率是 6%，但扣除稅盾後真正的負擔只有約 5.01%。也因為債務通常比股權便宜、又有稅盾，適度舉債能降低整體資本成本、提升股東回報 —— 這是企業保留一定負債的財務理由。但稅盾好處有上限，負債過高會推升破產風險，得不償失。
案例二：資本結構不同，WACC 卻可能相近
甲公司偏保守：權益權重 70%、權益成本 11%，債務權重 30%、稅前債務成本 5%，稅率 16.5%。WACC = 0.7 ×11% + 0.3 ×5% ×(1 − 0.165) = 7.7% + 1.25% ≈ 8.95%。
乙公司偏進取：權益權重 40%、權益成本 13% (槓桿高、股東要求回報更高)，債務權重 60%、稅前債務成本 7% (借得多、利率也較高)，稅率 16.5%。WACC = 0.4 ×13% + 0.6 ×7% ×(1 − 0.165) = 5.2% + 3.51% ≈ 8.71%。
有趣的是，儘管兩者資本結構天差地別，WACC 卻相當接近 (8.95% vs 8.71%)。這說明高槓桿雖然靠便宜債務壓低了加權成本，但同時推高了股權與債務各自的成本，兩股力量互相抵消。這也印證了資本結構理論的核心：WACC 對槓桿的反應並非單調下降，存在一個最適區間。
常見問題
What is the tax shield, and why does the formula use (1 - tax rate)?
Interest is deductible before tax, so each dollar of interest reduces taxable profit and saves tax of tax rate x interest. The after-tax cost of debt is Rd x (1 - tax rate). That is why WACC multiplies Rd by (1 - tax rate) — debt is cheaper after tax, one reason firms keep some leverage.
What is WACC used for?
It is the discount rate in DCF valuation and the hurdle rate for investment — projects must return above WACC to create value (positive NPV). It also benchmarks the cost of funding new projects.
Why does more debt not necessarily lower WACC?
Because more debt raises financial risk, so shareholders demand a higher Re and lenders charge a higher Rd. The cheaper-debt benefit is offset by the pricier equity and debt. The lowest WACC occurs at an 'optimal capital structure', not at maximum leverage.
Why use market values instead of book values for weights?
WACC is the current cost of capital, so it should use market values (equity market cap, debt market price) — reflecting today's true financing mix and costs. Book values are historical accounting figures and distort WACC when market prices move. If market values are unavailable, book values are a rough proxy, but state the assumption.
What is the Hong Kong tax rate to use?
Hong Kong's standard corporate profits tax (Profits Tax) is 16.5% (8.25% on the first HK$2 million of profits for eligible corporations). Use the applicable rate in the (1 - tax rate) term. The tax shield only applies to deductible interest under Hong Kong tax rules — confirm deductibility with a tax professional.
相關工具
參考資料
內容審核：香港計算器財經團隊。計算邏輯與公式參考香港金融管理局（HKMA）及投資者及理財教育委員會（IFEC）之個人理財計算指引，結果僅供參考，實際以相關機構公佈為準。