Cost of capital and capital structure
For a debt-and-equity capital structure, WACC = equity weight × cost of equity + debt weight × pretax cost of debt × (1 - tax rate). Use consistent market-value weights and match the rate to the project’s risk and cash-flow basis.
Calculate WACC without mixing inputs
- After-tax debt cost
- Pretax debt cost × (1 - applicable tax rate).
- Weighted return
- Equity weight × equity cost + debt weight × after-tax debt cost.
- Weight check
- Debt and equity weights must sum to 100% in this two-source model.
Use consistent target market-value weights. The debt adjustment assumes usable interest deductions; match the rate to project risk.
See the decision
Build the weighted rate
Target market-value weights are 60% equity and 40% debt. Cost of equity is 12%, pretax debt cost 7%, and the applicable tax rate 25%. Assume interest deductions are fully usable.
Components of the weighted rate
WACC: 9.30%
- Equity contribution7.20 pp
- After-tax debt contribution2.10 pp
The modeled WACC is 9.30%. Apply it only to cash flows with compatible risk and tax assumptions.
How to apply these ideas
1. Choose weights
Use the stated target or market-value capital proportions.
Confirm the debt and equity weights add to 100% and refer to the same capital base. Book balances can differ materially from market values. If the question specifies a target structure, use that consistently instead of mixing it with historical balances.
Watch for: Using face value for one weight and market value for the other without justification.
2. Price the components
Use required returns, not historical funding costs by default.
The cost of debt is the relevant current borrowing yield under the model; the cost of equity reflects the investors’ required return. A bond’s old coupon can differ from its current cost of capital. Add other funding components if the facts require them.
Watch for: Equating the coupon on old debt with today’s borrowing cost.
3. Apply the tax shield
Adjust debt cost only when the assumed tax benefit is usable.
Under the standard simplified model, interest deductibility reduces debt cost by the tax rate. Do not tax-adjust the equity return with the same formula. State restrictions or a lack of usable deductions when the problem includes them.
Watch for: Multiplying the entire WACC by one minus the tax rate.
4. Check suitability
A company-wide WACC is not automatically the rate for every project.
A project with different operating risk or leverage assumptions may need a different required return. More debt can also raise both debt and equity costs. A sensitivity table should change the relevant component rates when those assumptions change.
Watch for: Holding all rates fixed and declaring maximum debt optimal.
Study the topic
Cost of capital and capital structure: explanations and applications
Derive capital weights and component returns, calculate WACC and test how financing assumptions change the answer. Corporate finance remains the parent; this lesson is not a separate investment-appraisal guide.
By MiloPrep editorial team. Original teaching explanations and illustrative cases; official curriculum references are listed below.
Derive each input before inserting it into WACC
Market-value equity equals shares outstanding multiplied by share price. Market-value debt uses the relevant market value, not automatically face value. Add the included sources to establish the capital base, then divide each source by that base. If the case specifies target proportions, use the target consistently. Mixing target equity with historical debt can create weights that do not describe any actual structure.
For common equity, CAPM estimates required return as risk-free rate plus beta times the market risk premium. A constant-growth dividend model instead uses next-period dividend divided by current price plus sustainable growth, under its restrictive assumptions. The current dividend is not the next dividend when growth is assumed. These are models of required return, not guaranteed realized outcomes.
| Input | Check | Common error |
|---|---|---|
| Equity value | Shares × current price or stated target | Using par value |
| Debt cost | Relevant borrowing yield under the model | Using an old coupon without justification |
| Equity cost | Required return consistent with the model | Calling retained earnings free |
| Tax adjustment | Debt deduction is usable under the facts | Applying the shield to equity |
| Weights | Same capital base and sum to 100% | Mixing book and market inputs |
Account for additional sources and issuance costs explicitly
If preferred stock is included, add its weight and required return rather than forcing it into the debt or common-equity category. Under a simple perpetual preferred-dividend model, cost is annual preferred dividend divided by the relevant price or net proceeds when the problem specifies an issuance-cost convention. The debt tax shield does not automatically apply to preferred dividends.
Flotation costs are a financing transaction cost. Their treatment depends on the model and question; adding them to the project’s initial cash requirement can be more transparent than permanently increasing every future discount rate. Follow the stated convention and avoid counting the same issuance cost in both the cash flows and the rate.
Change component returns when leverage changes their risk
Debt can create a tax benefit while increasing financial risk. As leverage rises, lenders may require a higher yield and equity holders a higher return. A sensitivity table should change those assumptions when the scenario says risk changes. The mechanical decline obtained by replacing expensive equity with cheap debt at fixed rates is not a complete capital-structure analysis.
Use WACC only for compatible cash flows and risk. Nominal rates belong with nominal cash flows; after-tax rates with the matching tax basis; a company-wide business-risk assumption may not fit a new, riskier activity. When a project differs materially, explain the need for an adjusted required return instead of choosing a cheaper financing source to make NPV positive.
Work it through
Worked example: Build the weighted rate
Target market-value weights are 60% equity and 40% debt. Cost of equity is 12%, pretax debt cost 7%, and the applicable tax rate 25%. Assume interest deductions are fully usable.
| Component | Weight | After-tax component cost | Contribution |
|---|---|---|---|
| Equity | 60% | 12.00% | 7.20 percentage points |
| Debt | 40% | 5.25% | 2.10 percentage points |
- 1
After-tax debt cost = 7% × (1 - 25%) = 5.25%.
- 2
WACC = 60% × 12% + 40% × 5.25% = 7.20% + 2.10% = 9.30%.
The modeled WACC is 9.30%. Apply it only to cash flows with compatible risk and tax assumptions.
Work through the alternatives
More worked cases: cost of capital and capital structure
Original teaching scenarios. Each case states its assumptions, shows the working and explains the limits of the conclusion.
Calculate WACC from market values and CAPM
There are 100,000 shares at $30 and debt market value of $2 million. Risk-free return is 3%, beta 1.2 and market risk premium 5%. Debt yield is 6%, tax rate 25%, and debt tax benefits are usable.
| Input or component | Calculation | Result |
|---|---|---|
| Equity market value | 100,000 × $30 | $3,000,000 |
| Equity required return | 3% + 1.2 × 5% | 9% |
| Equity contribution | 60% × 9% | 5.40 percentage points |
| Debt contribution | 40% × 6% × 75% | 1.80 percentage points |
| WACC | 5.40% + 1.80% | 7.20% |
- Total market capital is $5 million, giving 60% equity and 40% debt.
- Apply the tax factor only to the 6% debt return, then add the weighted contributions.
What the result means: The component estimates and weights are now derived rather than supplied as unexplained inputs.
More debt can produce a higher weighted rate
Compare two illustrative target structures. At 40% debt, debt cost is 6% and equity cost 9%. At 60% debt, those costs rise to 9% and 13%. Tax is 25% and benefits are usable in both scenarios.
| Scenario | After-tax debt contribution | Equity contribution | WACC |
|---|---|---|---|
| 40% debt / 60% equity | 1.80 percentage points | 5.40 percentage points | 7.20% |
| 60% debt / 40% equity | 4.05 percentage points | 5.20 percentage points | 9.25% |
- First structure: 0.4 × 6% × 0.75 + 0.6 × 9% = 7.20%.
- Second structure: 0.6 × 9% × 0.75 + 0.4 × 13% = 9.25%.
What the result means: The tax shield does not offset the assumed increase in required returns. These are illustrative scenarios, not a claim that every firm has this cost curve.
Practice this topic
Check your understanding
Try each short application before opening its explanation. These are original learning checks, not official exam questions or a score prediction.
1. Current share price is $50, next year’s dividend $2 and constant expected growth 4%. What return does the constant-growth dividend model imply?
Show answer and explanation
8%.
2/50 + 4% = 8%. This relies on the model’s stable-growth and dividend assumptions.
2. Preferred shares pay a perpetual $6 annual dividend and sell for $75. Ignore issue costs. What is the simple preferred-stock cost?
Show answer and explanation
8%.
6/75 = 8%. Do not apply the standard debt interest tax adjustment to this dividend.
Questions about cost of capital and capital structure
- Why not use the cheapest source of funding as the discount rate?
- A project uses capital whose opportunity cost reflects business and financing risk. Selecting only a cheap debt rate can make a risky project appear attractive without compensating equity investors.
- Can a negative NPV become positive if more debt is added?
- Only if the economic assumptions and appropriate discount rate actually change. Changing spreadsheet weights while ignoring higher risk or funding costs does not demonstrate value creation.
Sources and scope
Curriculum mapping: Part 2, Section B, Corporate Finance. The official outline assigns 20% to the entire domain, not to this individual technique. These are selected study notes, not complete curriculum coverage.
Official curriculum references establish scope and expected skills. The explanations, diagrams and numerical cases are independent teaching material from MiloPrep, not official questions or source-validated solutions. Curriculum references checked 2026-10-10.
- IMA Content Specification Outlines (effective September 2024) ↗
Supports: Part 2, Section B, beginning on printed page 10. Domain names, weights and curriculum boundaries.
- IMA Learning Outcome Statements (effective September 2024) ↗
Supports: Part 2 B.2.q-v: WACC and component returns; printed page 20. Curriculum scope, not verification of this original worked example.
Independent US CMA study material. Not affiliated with or endorsed by IMA. This covers management accounting, not medical-assistant certification or the separate ICMAI qualification.