For each cell in Row 6 where you have an account enter the following formula: "=[Letter]2*[Letter]3/12" in the cell and hit the Enter key. For example, if you were going to enter the formula in B6, you would enter: "=B2*B3/12" and press the Enter key. Copy the formula in cell B6 and paste it into all other cells in Row 6 that have columns for accounts. Excel will automatically adjust the formula for any cells you copy/paste the formula into.  You will have previously entered all the credit card information in columns C, D, E, etc., depending on how many credit cards you have. The copied formula will calculate the new data from each column automatically. The annual interest rate is divided by 12 to arrive at a monthly interest charge. For this example of a $1,000 balance at 18% annual interest, the formula will return a monthly interest charge of $15. Once you have entered your data and the formulas have been calculated, you will see how much you are paying in interest on each card and how much your minimum payment of principal is each month. It's important to note the proportion of your payment that is allocated to interest and the amount that is being used to reduce your principal. You want your monthly payments to reduce your principal amount as much as possible, which you can do with a lower interest rate. To pay down your credit cards as quickly as possible, transfer your balances to cards with a lower interest rate or a zero percent introductory rate. See elsewhere in this article for ways to do this. Create a formula using the "SUM" function. The syntax is "=SUM(B6:E6)" where E6 represents the last cell in row 6 that has a number. This is how much you are paying in interest each month for all your credit cards. Keep in mind that the interest fee will change every month as you make payments on the balance.

Summary:
Calculate the monthly interest amount. Compare interest to principal payments. Total the sum of all monthly interest charges.