How to Calculate the APR on a Credit Card
To calculate the APR on your credit card, start by locating the annual percentage rate in your credit card agreement and seeing if it's fixed or variable. If your fixed rate is 12%, you will be charged a 12% annual interest rate on your balances. If you have a rate that varies, find the reference rate in your agreement and add the set margin number to calculate your APR. For example, if your reference rate is the US Prime rate of 3.25% and your margin is 5%, your APR would be 8.25%.
Read through your credit card agreement. You will receive the agreement when you get your credit card in the mail. A copy of the agreement will also be sent with your monthly statement. You'll also have access to the information online. The agreement has all of the factors used to calculate your Annual Percentage Rate (APR). Understand APR and why it is important to you. The Annual Percentage Rate (APR) is used to calculate the interest you owe on your credit card balance. If you understand how APR is applied to your account, you can take steps to reduce the interest cost on your balance. Review how interest is calculated on a loan balance. A credit card balance is a loan. The basic formula to calculate interest on a loan is (Interest rate) multiplied by (account balance) multiplied by (period of time). With credit cards, the APR is used for the interest rate variable in the formula. Verify your fixed annual percentage rate. Your credit card agreement may specify a fixed rate of interest you are charged each year. While the rate may be fixed, read your agreement carefully. You may find that the fixed rate is not guaranteed for the entire time you use the credit card. An interest rate change may be triggered by a change in bank interest rates, or triggered by some other event. Assume that your fixed rate is 12% per year. You will be charged a 12% annual rate of interest on your credit card balances. How the balance is calculated can vary. Review your variable APR. Many credit cards have an interest rate that varies. The variable rate changes based on a reference rate. The formula for your variable APR is (Margin) + (Reference rate). A common reference rate for a variable APR is the US Prime Lending Rate, or Prime Rate. The Prime Rate is the rate charged to a bank's best customers- those with the highest credit ratings. The reference rate changes over time. The Prime rate, for example, changes with movements in other US lending rates. Your credit card agreement will explain how often the reference rate can change in a given year. The margin is a set number that doesn't change. That set number is always added to the reference rate. Calculate a variable APR. Assume that your reference rate is the US Prime rate, which is 3.25%. Your margin is 5%. Your variable APR is (3.25% + 5% = 8.25%). Say that the Prime rate declines to 2%. Your new variable APR would be (2% + 5% = 7%). Assume that Prime rate increases to 5%. Your new variable APR would be (5% + 5% = 10%). Read your credit card agreement to see if there is a cap on the variable APR you can be charged. In some cases, there is a cap on how much your rate can be increased in a given year. Use monthly interest formula. The formula is the (Daily periodic rate) X (Account balance) X (Number of days in the billing cycle). Each component of the formula can vary. Convert your APR into a Daily Periodic Rate (DPR). The formula is (ARP/ 365). If your ARP is 10%, your DPR is.027% (with rounding). If you have a variable APR, your DPR will change each time the annual rate is adjusted. For fixed APRs, your daily rate will stay the same. Determine your account balance. For most credit cards, the account balance used is the average daily account balance. The formula is (sum of each day's account balance)/ (number of days in the month). Assume that your balance is $1,000 for the first 15 days of the month, and $2,000 for the last 15 days of a 30 days month. Your average daily account balance is [($1,000 X 15 days) + ($2,000 X 15 days)/ (30 days in the month)]. That equals ($15,000 + $30,000)/ 30, or $1,500. Several factors can generate a higher average higher daily account balance. Higher account balances increase the average. Also, a higher account balance for more days in a month increase the average. If you had a $10,000 account balance for 1 day, it would have less effect than a $10,000 balance for 20 days. Another method to calculate your average daily balance is (End of month account balance) less (Previous month's ending balance). The interest rate is calculated based on the difference. So, if your August balance was $10,000 and your September ending balance was $12,000, your average daily balance is ($12,000 - $10,000 = $2,000). Consider the number of days in your billing cycle. The credit card company will determine the last day of your billing cycle. For example, assume that billing cycle ends on the 20th of the month. Your September billing statement will be from August 21st to September 20th. Assume that your DPR is.027% and that your average daily account balance is $2,000. Say that there are 30 days in your billing cycle. Your interest for the month is (.027% X $2,000 X 30 = $16.44). The interest calculation would be increased by a higher APR, a higher average daily account balance, and by more days in the billing cycle. 
Read through your credit card agreement. Understand APR and why it is important to you. Review how interest is calculated on a loan balance. Verify your fixed annual percentage rate. Review your variable APR. Calculate a variable APR. Use monthly interest formula. Determine your account balance. Consider the number of days in your billing cycle. 