Estimating The Tax Benefits Of Debt

The standard approach to valuing interest tax shields assumes that full tax benefits are realized on every dollar of interest deduction in every scenario. The approach presented in this paper takes account of the possibility that interest tax shields cannot be used in some scenarios, in part because of variations in the firm's profitability. Because of the dynamic nature of the tax code (e.g., tax‐loss carrybacks and carryforwards), it is necessary to consider past and future taxable income when estimating today's effective marginal tax rate. The paper uses a series of numerical examples to show that (1) the incremental value of an extra dollar of interest deduction is equal to the marginal tax rate appropriate for that dollar; and (2) a firm's effective marginal tax rate (and therefore the marginal benefit of incremental interest deductions) can actually decline as the firm takes on additional debt. Based on marginal benefit functions for thousands of firms from 1980–1999, the author concludes that the tax benefits of debt averaged approximately 10% of firm value during the 1980s, while declining to around 8% in the 1990s. By taking maximum advantage of the interest tax shield, the average firm could have increased its value by approximately 15% over the 1980s and 1990s, suggesting that the consequences of being underlevered are significant. Surprisingly, many of the companies that appear best able to service debt (i.e., those with the lowest apparent costs of debt) use the least amount of debt, on average. Treasurers and CFOs should critically reevaluate their companies' debt policies and consider the benefits of additional leverage, even if taking on more debt causes their credit ratings to slip a notch.