How to Calculate Your Own Finance Charge

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Whenever you carry a credit card balance beyond the grace period (if you have one), you'll be assessed interest in the form of a finance charge. Fortunately, your credit card billing statement will always contain your finance charge, when you're charged 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 or you want to verify that your finance charge was billed correctly.

You can calculate finance charges as long as you know three numbers related to your credit card account: 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 billing cycles in the year) and that you have a $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 Shorter Billing Cycles

With most credit cards, the billing cycle is shorter than a month, for example, 23 or 25 days. If the number of days in your billing cycle is shorter than one month, 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

You might notice that the finance charge is lower in this example even though the balance and interest rate are the same. That's because you're paying interest for fewer days, 25 vs. 31. The total annual finance charges paid on your account would end up being roughly the same.

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 take into account transactions made on your credit card in the current or previous billing cycle.

Check your credit card agreement or the back of your credit card statement to determine how your finance charge is calculated and whether new purchases are included in the balance calculation.

The ending balance and previous balance methods are easier to calculate. The finance charge is calculated based on the balance at the end or beginning of the billing cycle.

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 every day 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.

You may not have a finance charge if you have a 0% interest rate promotion or if you've paid the balance before the grace period.

Frequently Asked Questions (FAQs)

What is the unpaid balance method?

The unpaid balance method is a way to calculate finance charges, but it's used less often than the average daily balance method. With this option, your finance charge is based on your unpaid balance. You calculate the interest on your unpaid balance, and add it to your total unpaid balance. Add on any new purchases and subtract any payments and credits.

What is a grace period?

A grace period for a credit card is the period between when your billing cycle ends and your payment is due. You may be able to avoid paying interest on purchases if you pay your balance in full by the payment due date. Cash advances typically don't have a grace period, and interest starts accumulating from the date of the cash advance.