Online Credit Card Interest Rate Calculator

Understanding your credit card interest can help you make better financial decisions. This calculator helps you estimate the interest you'll pay on your credit card balance, allowing you to compare different payment strategies and find the most cost-effective option.

How to Use This Calculator

To use this calculator, simply enter the following information:

Your current credit card balance
The annual percentage rate (APR) on your card
The number of days in the billing cycle
The number of days you've been using the card

Click "Calculate" to see your estimated interest charges. The calculator will show you the total interest, the average daily balance, and the interest rate used in the calculation.

Formula Used

The calculator uses the following formula to calculate interest charges:

Interest = (Average Daily Balance × Daily Interest Rate × Number of Days in Billing Cycle) / 365

Where:

Average Daily Balance = (Previous Balance × Number of Days in Billing Cycle + Current Balance × Number of Days Used) / Number of Days in Billing Cycle
Daily Interest Rate = Annual Percentage Rate (APR) / 365

Worked Example

Let's say you have a credit card with a $1,500 balance, a 17.99% APR, and a 30-day billing cycle. You've been using the card for 15 days.

Calculate the average daily balance:
- Previous balance: $1,500
- Current balance: $1,500
- Average daily balance = ($1,500 × 30 + $1,500 × 15) / 30 = $1,500
Calculate the daily interest rate:
- Daily interest rate = 17.99% / 365 ≈ 0.04924%
Calculate the interest:
- Interest = ($1,500 × 0.0004924 × 30) / 365 ≈ $1.68

In this example, you would pay approximately $1.68 in interest for the month.

Interpreting Results

The calculator provides several key pieces of information:

Total Interest: The total amount of interest you'll pay for the billing period.
Average Daily Balance: The average balance used to calculate interest.
Interest Rate: The daily interest rate used in the calculation.

Use this information to compare different payment strategies and find the most cost-effective option. For example, paying off your balance in full each month can help you avoid interest charges altogether.

Remember that credit card interest rates can vary significantly between cards. Always compare rates before choosing a card.

Frequently Asked Questions

How accurate is this calculator?: The calculator provides an estimate based on the information you provide. For exact figures, check your credit card statement.
What is the difference between APR and interest rate?: APR (Annual Percentage Rate) is the total cost of credit, including any fees, while the interest rate is the portion of APR that represents the cost of borrowing.
Can I use this calculator for different types of credit cards?: Yes, this calculator can be used for any type of credit card, including personal, business, and rewards cards.
How often should I check my credit card interest?: It's a good idea to check your interest charges at least once a month, or whenever you make a significant purchase.
What can I do to reduce my credit card interest?: You can reduce your interest by paying off your balance in full each month, transferring balances to a card with a lower APR, or negotiating with your credit card company for a lower rate.

About this calculator

Written by Calculator Editorial TeamPractical calculator research and UX writing

Reviewed by Practical Tools ReviewFormula logic, assumptions, and usability checks

Updated June 25, 2026. Formulas, assumptions, and limitations are shown directly on this page.

Formula and Source

The calculator uses standard credit card interest calculation methods. For more information, you can refer to the Federal Reserve's guidelines on credit card interest rates.