Credit Card Interest Calculator
What a card balance costs in interest this month, each day and over a year.
Good to know. At 22% APR, a balance left alone grows by about a quarter in a year. Interest charges interest too.
Not financial advice. Uses only the numbers and rates you enter — no live data.
How it works
In plain words
Divide the APR by 365 to get the daily rate, and multiply it by the balance and the number of days in the billing cycle. Left unpaid, the interest is added to the balance each day and starts earning interest itself.
The formula
daily rate = APR ÷ 365 interest = balance × daily rate × days effective rate = (1 + APR ÷ 365)³⁶⁵ − 1
Worked example
A 3,000 balance at 22% APR: the daily rate is 0.22 ÷ 365, about 0.06%, so a 30-day cycle costs 3,000 × 0.22 ÷ 365 × 30 = 54.25 in interest. Left unpaid for a year, the balance would grow by about 24.60%.
Sources
- Consumer Financial Protection Bureau, credit card interest and the daily periodic rate Issuers charge interest using a daily rate, the APR divided by 365 or 360, on the average daily balance.
- Brealey, Myers and Allen, Principles of Corporate Finance, chapter 2 The standard formulas for loan payments, compounding and present value, the same as a spreadsheet’s PMT and FV.
Sources checked October 3, 2026. 4 automated tests check its answers on every change.
About the credit card interest calculator
Credit card interest looks like one yearly number, the APR, but it is charged by the day. The issuer divides the APR by 365 (some use 360) to get a daily periodic rate, applies it to your balance each day, and adds it up at the end of the billing cycle.
That is why the days matter: a 31-day cycle costs a little more than a 28-day one at the same balance. Issuers usually use your average daily balance, so a payment made early in the cycle saves more than the same payment made just before the statement.
If you pay the statement balance in full by the due date, most cards charge no interest on purchases at all: the grace period. Interest starts once you carry a balance, and it keeps running on new purchases too until the card is paid off in full.
Left unpaid, interest is added to the balance and earns interest of its own, so the effective rate is higher than the APR. At 22% the balance grows by about 24.6% in a year.
To see how long a balance takes to clear at a given payment, use the credit card payoff calculator. Moving the balance to a 0% card can stop the interest for a while; the balance transfer calculator weighs the fee against the saving.
Questions people ask
How is credit card interest calculated?
Daily: the APR divided by 365, times the balance, for each day of the billing cycle. At 22% APR, a 3,000 balance costs about 1.81 a day.
Why is my interest charge different each month?
Billing cycles have different lengths, and the charge uses your average daily balance, which changes with every purchase and payment.
How do I avoid paying interest?
Pay the full statement balance by the due date each month. Most cards then charge no interest on purchases. Cash advances usually have no grace period.
What is the difference between APR and the effective rate?
The APR is the yearly rate before compounding. The effective rate includes interest on interest, so it is what an unpaid balance actually grows by in a year.