Credit card interest calculator
Credit card interest is not charged monthly in one lump. Your APR is divided by 365 into a daily periodic rate, that rate is applied to each day's balance, and the daily charges are added up at the end of the cycle. This is why paying earlier in the cycle costs less than paying on the due date.
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How the number is worked out
The daily periodic rate is the APR divided by 365. Some issuers use a 360-day basis, which makes each day marginally more expensive; CardLab supports both.
The rate applies to the balance on each individual day, so a purchase on day 2 is charged for more days than the same purchase on day 28.
None of this applies while your grace period is intact. Pay each statement balance in full and new purchases carry no interest at all, whatever the APR says.
Common questions
How much interest will I pay on a $5,000 credit card balance?
At 24.99% APR, $5,000.00 costs about $3.42 a day, or roughly $102.70 over a 30-day cycle. That is the cost of carrying it for one month; the total depends on how fast you pay it down.
Why was I charged interest when I paid my balance?
Usually residual interest. If you carried a balance last cycle, interest kept accruing between the statement date and the day your payment landed, so a charge appears on the next statement even though you paid in full. Paying in full again clears it and restores the grace period.
Does a 0% APR card really charge no interest?
On the balances the promotion covers, yes, until it expires. Purchases may be excluded if the offer is for balance transfers only, and cash advances are almost never included. Deferred-interest offers are different again: miss the deadline and the whole accrued amount is charged retroactively.