Credit Card Interest Calculator

Estimate Interest for a Billing Cycle

Credit Card Interest Formula

Formula

\[ \begin{aligned} \overline{B} &= \frac{\sum_{d=1}^{D} B_d}{D} \\ I &= \overline{B}\times\frac{a}{365}\times D \end{aligned} \]

Where:

$I$ — estimated interest charged for the billing cycle ($)
$\overline{B}$ — average daily balance ($)
$B_d$ — balance on day $d$, after any transactions entered for that day ($)
$D$ — number of days in the billing cycle
$a$ — APR expressed as a decimal; for example, 24.99% is 0.2499

The calculator divides the APR by 365 to obtain a daily rate, then applies it to each day’s balance. If you enter an average daily balance directly, it uses that amount without calculating individual daily balances.

Credit Card Interest – Calculation Example

Example: using the default average daily balance of $1,000, a 24.99% APR, and a 30-day billing cycle:

\[ I = \$1{,}000\times\frac{0.2499}{365}\times30 \approx \$20.54 \]

Result: estimated interest charged this cycle is $20.54. The daily periodic rate is approximately 0.06846575%, and the simple interest estimate for a full year at the same balance is $249.90.

How to use the calculator

Choose a Balance method. Select Enter average daily balance if you already know that figure, or Calculate from transactions to build it from a starting balance and changes during the cycle.
Enter the Annual percentage rate (APR) shown for the balance you want to estimate. Enter 24.99 for 24.99%, not 0.2499.
Enter Days in billing cycle as a whole number. Use the length of the particular statement period you are estimating, which may differ from one month to the next.
If you chose the average-balance method, enter Average daily balance. If you chose transactions, enter Balance at start of cycle and add 1 to 10 lines under Transactions, each in the form day; amount. For example, 10; 200 adds a $200 purchase on day 10, while 20; -100 records a $100 payment on day 20.
Read Interest charged this cycle for the main estimate. Check Average daily balance if you entered transactions, and use the comparison table to see what the same balance and cycle length would cost at other APRs.

What credit card interest measures

Credit card interest is the cost of carrying a balance. This calculator estimates that cost by applying a daily periodic rate—APR divided by 365—to the balance for each day of a billing cycle. A higher APR, a higher daily balance, or more days in the cycle increases the estimated charge.

The average daily balance captures the timing of purchases and payments. In transaction mode, a change takes effect at the start of the day you enter. With the default transaction entries, a $1,000 starting balance becomes $1,200 on day 10 and $1,100 on day 20. Over 30 days, the average daily balance is $1,103.33 when displayed to cents, and the estimated interest is $22.66. The calculator uses the unrounded average for its interest calculation.

The Interest per year at the same balance result is the average daily balance multiplied by APR. It is a simple-interest comparison for a balance held constant for 365 days, not a prediction of future statements. The separate Effective annual rate, monthly compounding shows $(1+a/12)^{12}-1$; it illustrates the effect of monthly compounding and is not used to calculate this cycle’s interest. For a broader explanation of how interest grows when it is added to a balance, see the Compound Interest Calculator.

Assumptions and practical limits

This is an estimate, not a reconstruction of your card issuer’s statement. It assumes one APR applies to the entire entered balance and uses a fixed 365-day divisor. Cards may treat purchases, cash advances, and balance transfers differently; promotional rates, fees, grace periods, and the issuer’s posting or rounding rules can also change the amount charged. If a grace period means no purchase interest is due, the calculator’s positive estimate does not override that.

In transaction mode, enter days from 1 through the specified cycle length. Purchases are positive and payments are negative. The calculator does not allow a daily balance below $0, so check a payment that exceeds the outstanding balance. For the most useful estimate, use transaction dates as they affect the balance rather than assuming every payment occurred on the final day.

Comparing APRs here holds the average daily balance and cycle length fixed; it does not account for changes in spending or repayment. If you are weighing a replacement loan against card debt, the Debt Consolidation Payment Calculator can help you examine the proposed payment separately.

Frequently Asked Questions

How can I check my credit card interest by hand?

Group together consecutive days when your balance was unchanged. Multiply each balance by the number of days it applied, add those amounts, then divide by the total days in the cycle to find the average daily balance. Multiply that average by the APR as a decimal and by the cycle length divided by 365. Your statement may differ because of grace periods, separate rates, or issuer rules.

Why does my credit card statement show no interest even though I had a balance?

A purchase grace period may let you avoid interest by paying your statement balance in full by the due date. A balance can therefore appear on a statement without generating purchase interest. Grace periods typically do not apply to cash advances, and carrying a balance can affect the grace period on new purchases. Check your card agreement for the rules that apply to you.

Why is my average daily balance different from my statement balance?

Your statement balance is a snapshot at the end of the billing cycle; your average daily balance reflects the amount owed throughout it. A large purchase near the end can raise the statement balance while affecting only a few days of the average. Conversely, a payment near the end may lower the statement balance without undoing interest accumulated earlier in the cycle.

Does making only the minimum payment stop credit card interest?

Usually not. A minimum payment generally keeps the account current, but interest can continue to accrue on the unpaid balance when a grace period does not apply. Paying more typically reduces future interest because less debt remains outstanding. Payment allocation can also matter when balances have different APRs, so consult your statement and card agreement rather than relying on a single-rate estimate.

What is a good credit card APR?

There is no single APR that is good for everyone: offers depend on creditworthiness, card features, and market conditions. Compare the rate you actually qualify for, including when an introductory rate ends, against other available offers. If you always pay purchases in full within the grace period, fees and benefits may matter more than the purchase APR. Review the offer’s terms before applying.

Can I use one APR for purchases, cash advances, and balance transfers?

Only if those balances actually have the same rate and interest rules. Cash advances often have a different APR and may accrue interest immediately; balance transfers may have promotional rates or transfer fees. For a rough estimate, calculate each balance category separately at its applicable rate, then add the interest estimates. Your issuer’s payment allocation and posting rules can still make the statement amount differ.

Is a 24% APR the same as 2% interest every month?

Not necessarily. Dividing 24% by 12 gives 2%, but many cards calculate interest using a daily rate and the number of days in each billing cycle. A 28-day cycle and a 31-day cycle can therefore produce different charges on the same balance. APR also does not tell you whether interest is currently due; a grace period or promotional terms may change that.