Many methods are used to calculate the fair value of a company. Among the methods most commonly used is the method of discounted cash flow, where the expected cash flows of the company are discounted by a rate to find an estimated value of the company. This rate, known by WACC, is a weighted average cost of equity and debt. In Brazil, however, there is a peculiarity in the legislation that affects the method of calculating the WACC: the Interest on Equity (IOE). Created in 1996, this factor allows the distribution of part of the profit discharged to shareholders as payment for the use of equity. The Brazilian law allows that this amount could be deducted from the base for calculating the income tax for the company, equivalent to interest paid to creditors. This peculiarity of Brazilian legislation impacts directly on the value of the weighted average cost of capital and therefore on the estimated fair value of the companies that make use of the IOE. Given that the traditional method of valuation of companies is not perfectly suited to Brazilian companies that use the distribution of interest on equity, it is necessary to adapt the traditional formula for calculating the WACC, considering the tax benefit of the IOE. Thus, this study aims to adapt the formula for calculating the weighted average cost of capital for the reality of Brazilian companies, in addition to demonstrate the impact of this change in calculating the estimated value of an active company in the country. The general objective of this work will test a model for calculating the weighted average cost of capital (WACC) in the Brazilian legal and accounting framework, considering the impact of the use of Interest on Equity. The specific objectives are the adequacy of the formula for calculating the WACC (including the IOE), the calculation of the value of a company with the traditional WACC, the calculation of the value of the same company with the adapted WACC and for last, the comparison of the value of the company found in both methods. To this end, we used the valuation report of Aracruz Celulose, held by a large investment bank. This company was chosen because it distributes interest on equity regularly. From this report, which used the traditional formula for calculating the WACC fair value of the company, we recalculate the fair value of the stock of that company, using the proposed formula. We then compared the values found by both methods, where was found that the value of the company was greater in the method proposed than in the traditional method. It was concluded that the distribution of interest on equity will be advantageous only in cases where the rate of income tax of the company exceeds the rate of income tax of the shareholders.