To determine the optimal capital structure using WACC, calculate the weighted average cost of capital at a series of debt-to-equity ratios, re-estimating the cost of debt and re-levering beta at each step, and pick the mix that produces the lowest number. WACC is the minimum return a company must earn to satisfy both its lenders and its shareholders, so driving it to its floor maximizes firm value. The work is mostly mechanical once the inputs are in hand; the judgment lies in modeling how borrowing costs and equity risk shift as leverage rises.
Inputs You Need Before You Start
Four numbers feed every version of the calculation.
- Market value of equity: current share price multiplied by shares outstanding, both pulled from the latest annual report or any major financial data platform.
- Market value of debt: the trading price of publicly traded bonds, or book value from the balance sheet if the debt isn’t traded.
- Corporate tax rate: 21 percent federal in the United States, plus a state corporate tax that runs roughly 1 to 12 percent depending on jurisdiction, so the combined effective rate sits above 21 percent.
- Risk-free rate: the yield on 10-year U.S. Treasury notes is the common choice, around 4.1 percent in early 2026. The Treasury publishes daily yield curves from one month out to 30 years.1Federal Reserve Economic Data. Market Yield on U.S. Treasury Securities at 10-Year Constant Maturity2U.S. Department of the Treasury. Daily Treasury Rates
For long-lived assets or whole-company valuations, some analysts use the 20-year or 30-year bond instead; recent 20-year issues came in near 4.6 percent and 30-year bonds near 4.75 percent.3TreasuryDirect. Treasury Bonds Match the maturity to the duration of the cash flows you’re discounting.
After-Tax Cost of Debt
The pre-tax cost of debt is the yield to maturity on the company’s outstanding bonds or the interest rate on its bank loans. Interest is deductible from taxable income under the federal tax code, which makes borrowing effectively cheaper than the headline rate.4Office of the Law Revision Counsel. 26 USC 163 – Interest Multiply the interest rate by (1 − tax rate) to capture that benefit. A 6 percent coupon at a 26 percent combined tax rate becomes 6% × (1 − 0.26) = 4.44 percent after tax. That single adjustment is why debt looks so attractive next to equity in a WACC model.
Synthetic Ratings for Unrated Companies
If the company has no credit rating or publicly traded bonds, build a synthetic rating from its interest coverage ratio: operating income divided by interest expense. Coverage above 8.5 times implies something close to AAA with a thin spread over the risk-free rate. Coverage of 1.5 times or less looks more like CCC, with a spread of 10 percentage points or more. Add the implied spread to the risk-free rate for a pre-tax cost of debt, then apply the tax adjustment.
How Section 163(j) Caps the Tax Shield
Many WACC analyses assume every dollar of interest produces a full deduction. Since 2018, Section 163(j) has capped business interest deductions at roughly 30 percent of adjusted taxable income, with the excess carried forward.5Internal Revenue Service. Questions and Answers About the Limitation on the Deduction for Business Interest Expense When your sensitivity analysis pushes annual interest above that threshold, the marginal tax benefit of additional borrowing drops to zero that year, and the WACC curve flattens or turns up sooner than a naive model predicts.
Two mitigants. For tax years beginning after December 31, 2024, legislation restored the add-back of depreciation, amortization, and depletion when computing adjusted taxable income, which lifts the cap for capital-intensive businesses. And small businesses with average annual gross receipts of roughly $31 million or less over the prior three years are exempt entirely; the figure is inflation-adjusted, with $31 million published for 2025 and the 2026 number not yet released.5Internal Revenue Service. Questions and Answers About the Limitation on the Deduction for Business Interest Expense
Cost of Equity
Equity holders don’t send invoices, but the return they expect is a real cost. The Capital Asset Pricing Model estimates it from three inputs: the risk-free rate, the stock’s beta, and the equity risk premium.
Beta measures a stock’s movement relative to the broader market. A beta of 1.0 tracks the market, above 1.0 is more volatile, below is calmer. Regression betas on individual stocks are noisy, so a cleaner method is bottom-up: take the average beta for the company’s industry, unlever each peer’s beta to strip out leverage effects, then re-lever to the company’s own debt-to-equity ratio using:
Levered beta = Unlevered beta × (1 + (1 − tax rate) × (Debt ÷ Equity))
Bottom-up beta is particularly useful for private companies or firms that recently changed their capital structure, where historical returns don’t reflect current risk.
The equity risk premium is the extra return investors demand for holding stocks over risk-free bonds. In early 2026, implied premiums for U.S. equities sit around 4 to 4.5 percent based on forward-looking models that back the premium out of current prices and expected earnings. Lock in one number for years and your WACC drifts out of date.
CAPM: cost of equity = risk-free rate + (beta × equity risk premium). At a 4.1 percent risk-free rate, a beta of 1.2, and a 4.3 percent equity risk premium, the cost of equity is 4.1% + (1.2 × 4.3%) = 9.26 percent.
Adding Preferred Stock to the Mix
Preferred shares pay a fixed dividend and sit between debt and common equity at liquidation. When they’re a meaningful slice of the balance sheet, leaving them out distorts the result.
Cost of preferred stock = annual preferred dividend per share ÷ current market price per share. A $5 dividend on an $80 share is 6.25 percent. Preferred dividends are not deductible, so no tax adjustment applies. The formula expands to three terms:
WACC = (E ÷ V) × cost of equity + (P ÷ V) × cost of preferred + (D ÷ V) × after-tax cost of debt
V is the total market value of the firm (equity + preferred + debt), and each numerator is the market value of that capital source.
Putting the WACC Together
Convert each source into a share of total firm value. A company with $600 million in equity and $400 million in debt has weights of 0.60 and 0.40, summing to 1.0. Multiply each weight by its cost and add. Using the earlier figures:
- Equity component: 0.60 × 9.26% = 5.56%
- Debt component: 0.40 × 4.44% = 1.78%
- WACC: 5.56% + 1.78% = 7.34%
That 7.34 percent is the hurdle rate; projects expected to earn less destroy shareholder value. WACC always falls between the cost of debt and the cost of equity. If yours lands outside that range, the weights or component costs are wrong.
Finding the Minimum Through Sensitivity Analysis
A single WACC tells you where the company stands today. Finding the optimum means running the calculation at a range of debt ratios and plotting the result.
Build a spreadsheet with debt ratios from 0 percent to 80 percent in 5- or 10-point increments. At each row:
- Set the equity weight at 1 minus the debt weight.
- Re-lever beta for the new debt-to-equity ratio and recalculate the cost of equity.
- Re-estimate the cost of debt. It does not stay flat. Lenders charge more as leverage rises, because the risk of nonpayment grows. The synthetic rating method handles this: higher leverage lowers the interest coverage ratio, worsens the implied rating, and widens the default spread.
- Apply the Section 163(j) cap. Once annual interest tops roughly 30 percent of adjusted taxable income, additional interest produces no immediate tax benefit.
Plot WACC against the debt ratio. The curve typically dips as initial debt replaces more expensive equity, bottoms out, then climbs as distress risk overwhelms the tax shield. The trough is the target.
The most common error is holding the cost of debt constant across all scenarios. A company financed 80 percent with debt borrows at rates far above a company at 30 percent, and ignoring that escalation makes the model recommend far too much leverage.
Why the Curve Turns Back Up
Two forces reverse the early benefits of leverage.
Equity investors demand more as the firm takes on debt. Their claim ranks behind lenders, so each added dollar of debt makes their position riskier. The re-levering formula captures this mechanically: debt-to-equity rises, levered beta rises, and the CAPM cost of equity follows.
Financial distress costs then enter the picture. Direct costs include legal fees, court costs, and restructuring advisors near or inside bankruptcy. Indirect costs often matter more: customers avoid shaky suppliers, key employees leave, suppliers demand cash on delivery, and the company passes on good investments because it can’t raise capital. Lenders price all of this into their rates, which is why the cost of debt also accelerates at high leverage.
The optimum sits where the marginal tax benefit of one more dollar of debt equals the marginal increase in distress costs. Past that point, borrowing more lowers firm value.
Flotation Costs When Issuing New Capital
Underwriting fees, legal expenses, and registration costs reduce what the company nets from a new issuance. If you’re modeling a company that will actively raise capital rather than evaluating an existing structure, adjust each component cost upward.
For equity, divide the unadjusted cost by (1 − flotation cost percentage). A 10 percent cost of equity with 3.5 percent issuance costs becomes 10% ÷ (1 − 0.035) = 10.36 percent. Debt adjusts similarly but carries a tax benefit on the issuance costs; equity flotation costs do not. For an established company analyzing its existing capital stack, flotation costs are already sunk and don’t belong in the ongoing WACC. The adjustment matters most for fast-growing firms that will keep tapping the markets.
A Worked Example
Take a mid-size manufacturer with no preferred stock, an unlevered beta of 0.85, a combined tax rate of 26 percent, a 4.1 percent risk-free rate, and a 4.3 percent equity risk premium. Four scenarios:
- 20% debt / 80% equity: levered beta = 0.85 × (1 + 0.74 × 0.25) = 1.01. Cost of equity = 4.1% + (1.01 × 4.3%) = 8.44%. Lender charges 5.0%; after-tax cost of debt = 3.70%. WACC = (0.80 × 8.44%) + (0.20 × 3.70%) = 7.49%.
- 40% debt / 60% equity: levered beta = 0.85 × (1 + 0.74 × 0.67) = 1.27. Cost of equity = 9.56%. Lender charges 5.5%; after-tax = 4.07%. WACC = (0.60 × 9.56%) + (0.40 × 4.07%) = 7.36%.
- 50% debt / 50% equity: levered beta = 0.85 × (1 + 0.74 × 1.0) = 1.48. Cost of equity = 10.46%. Lender charges 6.2%; after-tax = 4.59%. WACC = (0.50 × 10.46%) + (0.50 × 4.59%) = 7.53%.
- 60% debt / 40% equity: levered beta = 0.85 × (1 + 0.74 × 1.5) = 1.79. Cost of equity = 11.80%. Lender charges 7.5%; after-tax = 5.55%. WACC = (0.40 × 11.80%) + (0.60 × 5.55%) = 8.05%.
WACC bottoms around 40 percent debt in this run. Below it, the firm isn’t capturing enough of the tax shield; above it, the rising cost of equity and the widening credit spread wash out the benefit. A real analysis tests finer increments, applies the Section 163(j) cap at each leverage level, and stress-tests the beta and equity risk premium assumptions. The shape of the curve almost always looks the same: a decline, a trough, a climb. The trough is your target structure.