Quick answer
Interest for a billing cycle = average daily balance × APR ÷ 365 × days. A $3,000 balance at 19.99% for 30 days costs $49.29, and the monthly figure using APR ÷ 12 is $49.97.
Enter the average daily balance, APR and days in the cycle. The calculator returns interest for the period and the monthly estimate. Use this credit card interest calculator to check your own figures.
Related tools: Credit Card Payoff Calculator: Months to pay off a card balance with a fixed monthly payment. Debt Consolidation Calculator: Interest saved by replacing your debts with one fixed loan. Interest Rate Calculator: Annual interest rate implied by a loan amount, payment and term.
How to use the Credit Card Interest Calculator
- Enter average daily balance ($) (above 0).
- Enter aPR (%) (between 0 and 100).
- Enter days in billing cycle (a whole number, between 1 and 366). It starts at 30.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Average daily balance | A number, in $ | > 0 |
| APR | A number, in % | ≥ 0, ≤ 100 |
| Days in billing cycle | A whole number | ≥ 1, ≤ 366, starts at 30 |
Formula and method
Formula
interest = balance × APR ÷ 365 × days
IEEE-754 double precision; money shown to cents, rates to 4 decimals. Results are estimates, not financial advice.
Daily periodic rate (APR ÷ 365) applied to the average daily balance.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Average daily balance = 3000; APR = 19.99; Days in billing cycle = 30.
| Result | Value |
|---|---|
| Interest for the period | $49.29 |
| Monthly interest (APR ÷ 12) | $49.97 |
Example 2
Inputs: Average daily balance = 1500; APR = 24; Days in billing cycle = 31.
| Result | Value |
|---|---|
| Interest for the period | $30.58 |
| Monthly interest (APR ÷ 12) | $30.00 |
Example 3
Inputs: Average daily balance = 800; APR = 0; Days in billing cycle = 30.
| Result | Value |
|---|---|
| Interest for the period | $0.00 |
| Monthly interest (APR ÷ 12) | $0.00 |
The first line is the headline result (interest for the period). Below it the credit Card Interest Calculator lists monthly interest (APR ÷ 12), which describe the same case from other angles. IEEE-754 double precision; money shown to cents, rates to 4 decimals. Results are estimates, not financial advice.
Daily rate
The daily periodic rate is 19.99% ÷ 365 = 0.05477%. Issuers multiply it by each day's balance, so paying early in the cycle lowers interest. If you pay the full statement balance by the due date, there is usually no interest on purchases.
Why the two figures differ
Using 365 days over 30 days gives $49.29; APR ÷ 12 applied to a month gives $49.97. Actual statements follow the issuer's method, which may use 360 days.
How the result changes with average daily balance
Holding every other input at the first worked example, this table changes only average daily balance. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Average daily balance | Interest for the period | Monthly interest (APR ÷ 12) |
|---|---|---|
| 1500 | $24.65 | $24.99 |
| 3000 | $49.29 | $49.97 |
| 6000 | $98.58 | $99.95 |
| 12000 | $197.16 | $199.90 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Average daily balance: entering "0" shows "Average daily balance must be greater than 0." beside that field and no result.
- Days in billing cycle: entering "0" shows "Days in billing cycle must be at least 1." beside that field and no result.
Choosing between the Credit Card Interest Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Credit Card Payoff Calculator](/finance/credit-card-payoff-calculator/) | balance, aPR and monthly payment | Months to pay off |
| [Debt Consolidation Calculator](/finance/debt-consolidation-calculator/) | total balance, current average APR and total monthly payment now | Interest saved |
| [Interest Rate Calculator](/finance/interest-rate-calculator/) | loan amount, monthly payment and term | Interest rate (APR) |
| [Simple Interest Calculator](/finance/simple-interest-calculator/) | principal, annual rate and time | Interest |
| [Compound Interest Calculator](/finance/compound-interest-calculator/) | initial amount, annual interest rate and years | Future value |
Background: personal finance estimates
Financial calculators project from the numbers you give them. Long-range tools depend on guesses about returns and tax rates that are unlikely to hold exactly, payoff tools assume a fixed payment and rate, and tax or salary results depend on the tax year, filing status and country. Official calculators say their output is for reference only.
For decisions that matter, use conservative inputs and check the figures against your lender, plan documents or a qualified adviser.
Assumptions and limitations
- The monthly figure uses APR ÷ 12
Results are estimates for planning, not financial advice. Lender terms, fees, taxes and rounding rules can change real figures, so confirm important numbers with your lender or a licensed adviser.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for average daily balance and days in billing cycle to confirm this.
Frequently asked questions
What is APR on a credit card?
The yearly interest rate on unpaid balances.
What is a grace period?
Time before interest starts if you pay in full.
Does interest compound?
Yes, daily at most issuers.
What does the Credit Card Interest Calculator calculate?
Interest charged on a card balance for a billing period. It returns Interest for the period and Monthly interest (APR ÷ 12).
What formula does the Credit Card Interest Calculator use?
interest = balance × APR ÷ 365 × days. Daily periodic rate (APR ÷ 365) applied to the average daily balance.
What do I need to enter in the credit Card Interest Calculator?
You enter Average daily balance ($), APR (%) and Days in billing cycle.
How is the result rounded or limited?
IEEE-754 double precision; money shown to cents, rates to 4 decimals. Results are estimates, not financial advice. The monthly figure uses APR ÷ 12.
What is an example of a credit Card Interest result?
With average daily balance 3000, aPR 19.99 and days in billing cycle 30, the interest for the period is $49.29.
Can I rely on this calculator for a decision?
Results are estimates for planning, not financial advice. Lender terms, fees, taxes and rounding rules can change real figures, so confirm important numbers with your lender or a licensed adviser. For this calculator in particular: The monthly figure uses APR ÷ 12.
Related calculators
- Credit Card Payoff Calculator — Months to pay off a card balance with a fixed monthly payment.
- Debt Consolidation Calculator — Interest saved by replacing your debts with one fixed loan.
- Interest Rate Calculator — Annual interest rate implied by a loan amount, payment and term.
- Simple Interest Calculator — Interest earned or owed without compounding.
- Compound Interest Calculator — Future value with compound interest and regular contributions.
- Interest Per Day Calculator — Daily interest on a balance at an annual rate.
Sources
- Standard time-value-of-money relations (e.g. Brealey, Myers & Allen, Principles of Corporate Finance)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.