When analyzing a loan or an investment, it can be difficult to get a clear picture of the loan's true cost or the investment's true yield. There are several different terms used to describe the interest rate or yield on a loan, including annual percentage yield, annual percentage rate, effective rate, nominal rate, and more. Of these, the effective interest rate is perhaps the most useful, giving a relatively complete picture of the true cost of borrowing. To calculate the effective interest rate on a loan, you will need to understand the loan's stated terms and perform a simple calculation.