Weighted average cost of capital (WACC) is the average aftertax cost of a company’s various capital sources, including common stock, preferred stock, bonds and any other longterm debt. It is calculated by multiplying the cost of each capital source by its relevant weight, and then adding the products together to determine the WACC value.
Internal and external factors can cause problems for investors and analysts trying to assess the performance of a firm over time and in comparison with other firms in the industry. One external factor is the fluctuation of interest rates. The Federal Reserve Bank (the Fed) moderates longterm interest rates by targeting the federal funds rate  the interest rate at which one bank lends funds maintained at the Federal Reserve to another bank overnight.
The Fed attempts to align the effective federal funds rate with the targeted rate by making additions to or subtractions from the money supply through open market operations (the buying and selling of U.S government securities, or Treasury bills).
As interest rates are moderated, it can cause fluctuations in the riskfree rate, the theoretical rate of return for an investment that has no risk of financial loss. This can affect a firm's WACC because the riskfree rate is an important factor in calculating the cost of capital. As the interest rate on debt fluctuates, it can be challenging for a company to predict the future costs of capital. As a result, a company can end up with greater or lesser capital costs than expected because of fluctuations in interest rates. A firm's cost of debt must be updated frequently as the cost of debt reacts to fluctuations in interest rates.
Other external factors that can affect WACC include corporate tax rates, economic conditions, and market conditions.

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