Credit Card Interest Calculator

Understanding Credit Card Interest

Credit card interest can be one of the most significant financial burdens if not managed carefully. Unlike fixed installment loans, credit card debt is revolving, and interest is typically compounded daily. Understanding how card issuers calculate interest helps you make strategic decisions about your payments, potentially saving you thousands of dollars.

How is Credit Card Interest Calculated?

Most credit card issuers use a method called the **Average Daily Balance (ADB)** to compute the interest charged at the end of each billing cycle. Here is the step-by-step breakdown of how this calculation works:

• Daily Periodic Rate: The Annual Percentage Rate (APR) is divided by 365 (or sometimes 360) to determine the daily interest rate. $$\text{Daily Rate} = \frac{\text{APR}}{365}$$
• Daily Balance: Every day during the billing cycle, the issuer tracks your ending balance, adding new purchases and fees and subtracting payments or credits.
• Average Daily Balance: The daily balances for each day of the billing cycle are added together and divided by the number of days in the cycle. $$\text{ADB} = \frac{\sum \text{Daily Balances}}{\text{Days in Cycle}}$$
• Monthly Interest Charge: The Average Daily Balance is multiplied by the Daily Periodic Rate, then multiplied by the number of days in the billing cycle. $$\text{Monthly Interest} = \text{ADB} \times \left(\frac{\text{APR}}{365}\right) \times \text{Days in Cycle}$$

The Cost of Paying Only the Minimum

When you make only the minimum payment required by your credit card company, a large portion of that payment goes directly toward covering the accrued interest rather than reducing your principal balance. The remaining principal continues to accrue interest at the high APR, leading to a long, expensive payoff timeline.

If your payment is too small, it may not even cover the interest accrued during the month. When this happens, your balance will grow forever, creating an infinite debt spiral. Even accelerating your payments slightly (e.g., paying 1.5 times the minimum) can significantly reduce your payoff duration and total interest paid.

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