What is an 'Optimal Capital Structure'
An optimal capital structure is the best mix of debt, preferred stock and common stock that maximizes a company’s stock price by minimizing its cost of capital. In theory, debt financing offers the lowest cost of capital due to its tax deductibility. However, too much debt increases the financial risk to shareholders and the return on equity that they require. Thus companies have to find the point at which the marginal benefit of debt equals the marginal cost.
BREAKING DOWN 'Optimal Capital Structure'
The optimal capital structure is estimated by calculating the mix of debt and equity that minimizes the weighted average cost of capital (WACC), while maximizing its market value. The lower the cost of capital, the greater the present value of the firm’s future cash flows, discounted by the WACC. Thus, the chief goal of any corporate finance department should be to find the optimal capital structure that will result in the lowest WACC and the maximum value of the company (shareholder wealth).
What Mixture of Equity and Debt Will Result in the Lowest WACC?
The cost of debt is less expensive than equity, because it is less risky. The required return needed to compensate debt investors is less than the required return needed to compensate equity investors, because interest payments have priority over dividends and debt holders receive priority in the event of liquidation. Debt is also cheaper than equity, because companies get tax relief on interest, while dividend payments are paid out of aftertax income.
However, there is a limit to the amount of debt a company should have, because an excessive amount of debt increases interest payments, and the volatility of earnings and the risk of bankruptcy. This increase in the financial risk to shareholders means that they will require a greater return to compensate them, which increases the WACC — and lowers the market value of a business. So, the optimal structure involves using enough equity to mitigate the risk of being unable to pay back the debt — taking into account the variability of the business’ cash flow.
Companies with consistent cash flows can tolerate a much larger debt load, and will have a much higher percentage of debt in their optimal capital structure. Conversely, a company with volatile cash flow, will have little debt and a large amount of equity.
Changes in Capital Structure Send Signals to the Market
As it can be difficult to pinpoint the optimal structure, managers usually attempt to operate within a range of values. They also have to take into account the signals their financing decisions send to the market. A company with good prospects will try to raise capital using debt rather than equity, to avoid dilution and sending any negative signals to the market. Announcements made about a company taking debt are typically seen as positive news – which is known as debt signaling. Also, if a company raises too much capital during a given time period, the costs of debt, preferred stock, and common equity will begin to rise, and as this occurs, the marginal cost of capital will also rise.
To gauge how risky a company is, potential equity investors look at the debt/equity ratio. They also compare the amount of leverage other businesses in the same industry are using – on the assumption that these companies are operating with an optimal capital structure — to see if the company is employing an unusual amount of debt within its capital structure.
Another way to determine optimal debttoequity levels is to think like a bank. What is the optimal level of debt a bank is willing to lend? An analyst may also utilize other debt ratios to put the company into a credit profile using a bond rating. The default spread attached to the bond rating can then be used for the spread above the riskfree rate of an AAArated company.

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How does a company choose between debt and equity in its capital structure?
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How do you calculate the ratio between debt and equity in the cost of capital
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How does a company's capitalization structure affect its profitability?
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What is the difference between cost of debt capital and cost of equity?
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What is the formula for calculating weighted average cost of capital (WACC)?
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