In most cases, you're supposed to receive complete information about your loan before you close, including the APR (annual percentage rate), which includes all fees built into the loan. But in some scenarios you may not know the rate off the bat -- for instance, if you take over a payment from someone else. You can resolve the predicament using an online calculator, as long as you have all other details about the loan on hand, including the loan amount, term, and the amount of your monthly payment.

Use the eFunda calculator to determine the interest rate associated with your loan payment (see Resources). Choose "Scenario 2: Find the Interest Rate." Before you perform the calculation, eFunda provides you with a quick explanation of how the calculator determines the rate. After you key in your loan's amount, your monthly payment, and the total number of months your loan will last, click "Calculate." The calculator displays the interest rate, as well as the total principal and interest paid.

Determine the interest rate on the loan in question using the Your Money Page interest calculator (see Resources). This calculator says that it's specifically designed for an auto loan, but you can use it for just about any other similar type of installment loan. Enter the initial amount of the loan, your monthly payment, and your loan term in months. Then click "Calculate" to display the APR rate on the next screen. You need to accept cookies for the calculator to work on this website.

Calculate the annual rate on your loan payment with the simple tool at Intrepid Technology's website (see Resources). This calculator allows you to enter four of five loan variables to then calculate the fifth automatically. So, enter information into all the fields except the one for interest rate. When you are done, click "Compute" next to the interest rate field to display the APR.