Credit Card Interest Calculator

When to round up

Interest rounds to cents after the full calculation. Real statements may use daily balances that change with purchases, payments, credits, fees, grace periods, different APR categories, issuer-specific rounding, or a different day-count convention. Use the statement's average daily balance when available.

When to use this calculator

• Estimating interest for one balance and APR category over a billing period
• Converting APR into a daily periodic rate and period rate
• Comparing average-daily-balance, daily-compounding, and qualifying purchase-grace-period scenarios

Tips for better estimates

• Use the average daily balance for the relevant APR category from the statement when available.
• Keep purchases, cash advances, and transfers separate when they have different APRs.
• Select the grace-period scenario only after confirming the purchase grace period applies and its conditions were met.

How this calculator is reviewed

This page is checked for inputs, formulas, examples, assumptions, topic fit, and related links. For this calculator, the review also covers the correct balance and APR category, average daily balance, day count, selected method, grace-period conditions, separate fees, statement rounding, and issuer terms.

The sample result is covered by automated tests, and the page links to related calculators and, where available, supporting guides so readers can check the assumptions before acting. If a formula, label, or assumption looks off, send the page URL and your inputs through the contact page.

Worked example

Example inputs: Average daily balance or starting balance: $3500; APR: 24.99 %; Days in the estimate: 30; Additional fees: $0; Interest method: Average daily balance estimate. With those values, the calculator returns $71.89 estimated interest. This average daily balance estimate multiplies the entered balance by APR divided by 365 and by the entered days; use the average daily balance for the relevant APR category from your statement when available. Entered fees are shown separately rather than treated as interest.

Example scenarios

• A $3,500 average daily balance at 24.99% APR for 30 days produces about $71.89 in interest under the APR-divided-by-365 estimate.
• The same unchanged starting balance produces about $72.61 under the optional daily-compounding sensitivity, showing why the selected method matters.
• If the statement separates purchases, cash advances, or transfers by APR, run each category separately instead of blending them.

Quick reference chart

Common planning mistakes

Using a current balance in place of average daily balance, combining balance categories with different APRs, counting fees as interest, assuming a grace period applies, and treating the estimate as the issuer's statement calculation.