Optimal capital structure

Theoretically, enterprises should try to maintain a certain optimal capital structure, a perfect mix of financing (debt and equity), which results in the lowest possible weighted average cost of capital. At this combination of debt and equity, the stock price is at the maximum. Therefore, attainment of the optimal structure is in line with the main objective of the business, which is the maximization of wealth of the owners of the business. The optimal structure is also referred to as the target capital structure. However, it is important to note the optimal structure exists only in theory.

Theory does not yet offer a methodology that would allow firms’ financial managers to find the optimal capital structure. However, financial managers can determine the approximate optimal structure range, which is close to what they believe the optimal structure for the firm is.

As per above, an optimal structure maximizes the value of the firm. To find the value of the firm, we can use the following formula:


Which simplifies into:



V = is the value of the firm

EBIT = is earnings before interest and taxes (see the income statement for how it is calculated)

NOPAT = is the net operating profit after taxes (calculated by formula EBIT*(1-T)/ra)

ra = is the weighted average cost of capital (WACC)

If we assume that NOPAT is consistent, then the value of the firm is affected by WACC (ra, weighted average cost of capital). WACC is affected by both, the cost of debt and equity.

The cost of equity

The cost of equity is higher than the cost of debt and increases as financial leverage increases. This is because equity suppliers will demand higher return for increasing financial risk due to increasing financial leverage.

The cost of debt

The cost of debt initially is relatively low. The major reason for this is due to the fact that interest on debt is tax deductible. This tax deductibility of interest paid on debt is also commonly called the tax shield. However, as debt increases, at certain debt ratio lenders will begin to require higher and higher interest payments from the borrower. This is undertaken in order to compensate for increasing risk due to increasing financial leverage.

There are two other costs of debt that the firm needs to consider:

(1) Debt increases the probability of bankruptcy. This is because lenders can force the firm into bankruptcy if the firm cannot meet its financial obligations to the lender.

(2) Another aspect to consider is the agency cost. This refers to the fact that lenders usually protect themselves from increases in risk of the borrower by imposing different loan provisions, which place constraints on actions and choices of the firm. Such provisions commonly include, but are not limited to, minimum levels of liquidity to be maintained, limits on compensation of the executives and limitations on asset acquisitions.


As debt increases from a zero point onwards, WACC initially decreases to the theoretical optimal capital structure point. Thereafter, the increasing equity cost and increasing cost of debt causes WACC to start increasing again. Therefore the theoretical optimal capital structure is obtained at the point where the WACC is the lowest.

In other words, the theoretical optimal capital structure occurs at the point where the benefits from using debts are in equilibrium (in balance) with the costs of using debt. The optimal capital structure can also be seen as the balance between risk and return where the firm’s stock price is maximized.



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