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Financial Planning

Credit Card Interest Calculator

Compute one billing cycle’s credit card interest using the average daily balance method, including purchases and payments.

Your inputs

Results

Interest for this cycle

$49.04

Average daily balance
$2,983.33
Daily periodic rate
0.05479%
Balance at cycle end incl. interest
$2,549.04

✓Exact calculation — rounded only for display

Inputs are never added to links for financial, salary or health calculators.

What this calculator does

Most cards charge interest on the average of your balance over each day of the billing cycle. The timing of purchases and payments therefore changes how much interest you pay.

Formula and method

Average daily balance = Σ daily balances ÷ days in cycle Daily periodic rate = APR ÷ 365 (or 360) Interest = ADB × daily rate × days in cycle

Worked example

Calculated by this tool with its default inputs:

Inputs

  • Balance at start of cycle: 3000 (any currency)
  • Purchases during the cycle: 500 (any currency)
  • Day purchases post: 10
  • Payment during the cycle: 1000 (any currency)
  • Day payment posts: 20
  • Days in billing cycle: 30
  • APR: 20 %
  • Days per year used by issuer: 365

Results

  • Interest for this cycle: 49.04
  • Average daily balance: 2,983.33
  • Daily periodic rate: 0.05479%
  • Balance at cycle end incl. interest: 2,549.04

How to read the results

Paying earlier in the cycle lowers the average daily balance and therefore the interest.

Assumptions and limitations

  • If you pay the full statement balance by the due date, many cards charge no interest on purchases (grace period) — this tool assumes interest applies.
  • Compounding of daily interest within the cycle is ignored.

Financial results are estimates based on the inputs and assumptions shown. They are not financial advice; confirm figures with your lender, provider or a qualified adviser before making decisions.

Questions

What is the daily periodic rate?

It is your APR divided by the number of days the issuer uses for a year (usually 365). It is applied to each day’s balance.

Methodology and verification

Result type
Exact mathematical calculation (rounded only for display)
Formula version
1.0
Automated verification
3 reference and validation test cases, checked against independently computed values. The site cannot be built if any of them fail.
Editorial review
Awaiting human editorial review. The explanation was drafted with AI assistance; the formula and examples are verified by the automated tests above.

Privacy

Everything you enter is calculated in this browser and is not sent to Numerith or anyone else. Because this calculator can involve personal financial or health details, your inputs are never added to share links or saved to history. Privacy policy