When financing a new project, business, or operation, organizations can utilize both equity and debt to create a balanced weighted average cost of capital. One of the primary differences between equity and debt pertains to risk, and the impact that risk has on the cost of capital. To understand the role of debt, and how the cost of debt impacts overall cost of capital, the weighted average cost of capital equation is a useful data point:

${\displaystyle {\text{WACC}}={\frac {MV_{e}}{MV_{d}+MV_{e}}}\cdot R_{e}+{\frac {MV_{d}}{MV_{d}+MV_{e}}}\cdot R_{d}\cdot (1-t)}$

Calculating the Cost of Debt (R_d)

When an organization borrows capital from outside lenders, the interest on these loans is called debt. The variables involved in a debt transaction are fairly simple:

• K_d - The cost of debt (referred to as R_d above)
• R_f - The risk free rate
• T - Tax
• Credit risk rate - Or the risk premium

When these variables are plugged into a formula, it looks like this:

${\displaystyle K_{D}=(R_{f}+{\text{credit risk rate}})(1-T)}$

The risk -free rate (or R_f) is externally determined from the general market, and is described as the overall cost incurred due to the time value of money with no risk whatsoever involved. This is usually derived from a government treasury bond, as it is the most the investment asset in most markets with the lowest possible rate of risk. Tax rates are set externally as well, and are concrete. 

The credit risk rate is therefore the point of negotiation, and where a risk premium is attached to the debt instrument to compensate the investor in regards to a return (for the risk taken). As a general rule, the larger the debt is the higher the risk rate will be (as all other things being equal, a higher debt is harder to pay back). Debtors will also take into consideration the collateral available to the organization (i.e. a valuation of their assets).

Debt Compared to Equity

Determining the cost of debt is quite a bit easier than determining the cost of equity. The inputs to a debt assessment are tangibly determined, and consistent across the life of the agreement. It is worth noting that debt is generally less risky from the investor's point of view, as debt obligations are paid out before equity obligations if a business goes bankrupt.