Credit Card Interest Calculator

Billing-cycle interest tool

Credit Card Interest Calculator

Estimate an interest charge from a known average daily balance or reconstruct the balance across a billing cycle using dated purchases and payments.



Formatting only; no exchange-rate conversion.





Use the divisor stated in the card agreement.


Transactions below apply at the start of the selected day.

Transactions during the cycle

Optional. Combine transactions of the same type and day if more than four occurred.

How this credit card interest calculator works

Many credit cards calculate purchase interest with a daily periodic rate and a daily balance. This tool can use an average daily balance you already know or rebuild that average from an opening balance and up to four dated purchases or payments.

Daily periodic rate: DPR = APR ÷ day-count basis.

Average daily balance: ADB = sum of interest-bearing daily balances ÷ billing-cycle days.

Estimated interest: interest = ADB × DPR × billing-cycle days. This is equivalent to sum of interest-bearing daily balances × DPR.

The timeline mode applies each transaction at the start of its selected day. Payments reduce the balance and purchases increase it. A credit balance below zero is counted as zero for interest, while the actual negative balance is preserved in the ending-balance estimate.

Checked 30-day example

Start with $2,000 at a 24% APR using APR ÷ 365. Apply a $500 payment on day 11 and a $300 purchase on day 21.

  1. Days 1–10: $2,000 × 10 = 20,000 balance-days.
  2. Days 11–20: $1,500 × 10 = 15,000 balance-days.
  3. Days 21–30: $1,800 × 10 = 18,000 balance-days.
  4. Total = 53,000 balance-days; ADB = 53,000 ÷ 30 = $1,766.6667.
  5. DPR = 24% ÷ 365 = 0.0657534% per day.
  6. Interest = 53,000 × 24% ÷ 365 = $34.8493, displayed as $34.85.

The balance before interest is $1,800, and the estimated balance after adding interest is $1,834.85.

Average daily balance example

For an average daily balance of $1,500, a 20% APR, a 31-day cycle and a 365-day divisor: $1,500 × 20% ÷ 365 × 31 = $25.4795, or approximately $25.48.

Why the actual charge can differ

Your agreement controls the calculation. An issuer may use a different day-count basis, separate APRs for purchases, cash advances and balance transfers, different transaction posting dates, daily compounding, minimum interest, fees or promotional and grace-period rules. Purchases entered here are treated as interest-bearing from the selected day, which may overstate interest when a grace period applies.

Use the statement as the authoritative amount. This calculator is an educational estimate and does not determine the payment due, eligibility for a grace period, or whether interest will be waived.

How to use the timeline accurately

  • Enter the opening interest-bearing balance for day 1, not merely the current app balance.
  • Use posting dates if they differ from purchase or payment dates.
  • Combine same-day purchases and same-day payments when more than four transactions occurred.
  • Keep every amount in the same currency. The currency selector changes symbols and formatting only.

Limits and validation

APR is limited to 0%–100%, cycle length to 1–62 whole days and monetary inputs to one trillion. Transaction days must fall inside the selected cycle, and a day and positive amount must be supplied together. Empty, nonnumeric and negative required inputs are rejected. The calculation does not forecast future payoff, minimum payments, late fees or credit-score effects.

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