Your credit card or loan billing statement will always contain your finance charge (if you have one), so there’s not necessarily a need to calculate it on your own. But, knowing how to do the calculation yourself can come in handy if you want to know what finance charge to expect on a certain credit card balance. You can calculate finance charges as long as you know three numbers: the credit card (or loan) balance, the APR, and the length of the billing cycle.
Calculating Finance Charges the Simple Way
The simplest way to calculate a finance charge is
balance X monthly rate. For this example, we’ll say each billing cycle lasts a month, so there are 12 in the year and that $500 credit card balance with an 18% APR.
First, calculate the periodic rate by dividing the APR by the number of billing cycles in the year, which is 12 in our example. Remember to convert percentages to a decimal. The periodic rate is
.18 / 12 = 0.015 or 1.5%. The monthly finance charge is
500 X .015 = $7.50.
Calculating Finance Charges With Shorter Billing Cycles
With most credit cards, the billing cycle is shorter than a month, for example, 23 or 25 days. If you have a different billing cycle, calculate your finance charge like this:
balance X APR X days in billing cycle / 365.
Example: If your billing cycle is 25 days long, the finance charge for that billing period would be
500 x .018 X 25 / 365 = $6.16. The finance charge is lower than the first example because you’re paying interest for fewer days. It all evens out by the end of the year.
Variations in Credit Card Issuer Finance Charge Calculation Methods
The examples we’ve done so far are simple ways to calculate your finance charge, but still may not represent the finance charge you see on your billing statement. That’s because your creditor will use one of five finance charge calculation methods that takes into account transactions made on your credit card in the current or previous billing cycle. Read the back of your credit card statement to figure out which method your credit card issuer uses.
The adjusted balance method is slightly more complicated; it takes the balance at the beginning of the billing cycle and subtracts payments you made during the cycle.
The daily balance method sums your finance charge for each day of the month. To do this calculation yourself, you need to know your exact credit card balance everyday of the billing cycle. Then, multiply each day’s balance by the daily rate (APR/365). Add up each day’s finance charge to get the monthly finance charge.
Credit card issuers most often use the average daily balance method, which is similar to the daily balance method. The difference is that each day’s balance is averaged first and then the finance charge is calculated on that average. To do the calculation yourself, you need to know your credit card balance at the end of each day. Add up each day’s balance and then divide by the number of days in the billing cycle. Then, multiply that number by the APR and days in the billing cycle. Divide the result by 365.
Note that you may not have a finance charge in months that a 0% interest rate is in effect or if you've paid the balance before the grace period.