How to Calculate Credit Card Interest on A Purchase

What is Credit Card Interest?

Credit card interest is the cost of borrowing money through your credit card. It's calculated based on the balance you carry each month, the interest rate your card charges, and the length of time you carry that balance.

Most credit cards charge interest on both purchases and cash advances, though the rates may differ. The interest is typically calculated daily and added to your balance, which can lead to compounding interest over time.

How Interest is Calculated

The calculation of credit card interest typically follows these steps:

1. Determine your daily balance - this is your average daily balance for the billing period
2. Multiply the daily balance by your daily interest rate (APR divided by 365)
3. Sum the daily interest charges for the billing period
4. Add the interest to your previous balance to get the new balance

Most credit cards use the average daily balance method, which means your interest is calculated based on the average of your daily balances during the billing period.

Key Terms

APR (Annual Percentage Rate)

The annual interest rate your credit card charges on purchases and cash advances.

APY (Annual Percentage Yield)

The effective annual interest rate, taking into account compounding interest.

Daily Balance

The average balance carried each day during the billing period.

Grace Period

The time after your statement closes when you can pay your balance without interest.

Minimum Payment

The smallest amount you must pay each month to avoid penalties.

Step-by-Step Calculation

To calculate credit card interest manually, follow these steps:

1. Find your credit card's APR
2. Determine your daily balance (average daily balance)
3. Calculate the daily interest rate (APR ÷ 365)
4. Multiply the daily balance by the daily interest rate
5. Sum the daily interest charges for the billing period
6. Add the interest to your previous balance

For more accurate calculations, some cards use the average daily balance method, which considers the average of your daily balances during the billing period.

Example Calculation

Let's calculate the interest on a $1,000 purchase with a 18% APR over a 30-day billing period:

1. APR = 18% or 0.18
2. Daily interest rate = 0.18 ÷ 365 ≈ 0.000493
3. Daily interest = $1,000 × 0.000493 ≈ $0.493
4. Total interest for 30 days = $0.493 × 30 ≈ $14.79

In this example, you would pay approximately $14.79 in interest for carrying a $1,000 balance for one month.

How to Use This Calculator

Our credit card interest calculator makes it easy to estimate your interest charges:

1. Enter your purchase amount
2. Input your credit card's APR
3. Select the billing period length
4. Click "Calculate" to see your estimated interest

The calculator uses the average daily balance method to provide the most accurate estimate. Remember that actual interest charges may vary based on your specific credit card terms and billing cycle.