How credit card interest works

Card interest is a daily charge dressed up as an annual number. Your APR is divided by 365 to get a daily periodic rate, that rate is applied to your average daily balance every day, and the total lands on your statement once a month. Everything else — grace periods, minimum payments, residual interest — follows from that one mechanism. Every figure below is computed live from one worked example: $6,000 at 24.99%.

1. The daily rate is the real rate

24.99% ÷ 365 = 0.06847% per day. On $6,000 that is $4.11 every day, or $123.24 over a 30-day cycle. That per-day figure is the most useful number in card debt: it turns an abstract percentage into what today cost.

Daily interest calculator →

2. Each payment splits, and the split moves

A payment covers the cycle's interest first; the remainder reduces principal — the CFPB's amortization explainer describes exactly this. Paying $250.00 a month on our example, the first payment is $124.95 interest and $125.05 principal. By month 1 the split has flipped and most of your payment is finally principal. The final payment, month 34, is almost entirely principal — the whole schedule is just this split walking in one direction.

Month	Interest	Principal	Balance after
1	$124.95	$125.05	$5,874.95
6	$111.38	$138.62	$5,209.54
12	$93.13	$156.87	$4,315.02
34	$3.13	$150.09	$0.00

Total across the whole schedule: 34 payments, $2,403.22 of interest on $6,000 borrowed.

3. The monthly convention this site uses

Our engine accrues once a month at 24.99% ÷ 12 = 2.0825% and rounds to cents after every step. Issuers compound daily, which makes a real statement slightly worse than this convention shows — a few dollars over a typical payoff, never a different answer about how many months it takes. We disclose the approximation on every tool rather than implying statement-level precision we do not have.

4. The grace period is the escape hatch

Pay the statement balance in full by the due date and most cards charge no purchase interest at all. The corollary matters: carrying a balance does not help your credit. What helps is paying on time and keeping utilization low — and the utilization calculator computes that ratio without pretending to predict a score.

Monthly interest accrualconvention — interest per month = balance x (APR / 12). Verified 2026-07-31 against CFPB — How does my credit card company calculate the amount of interest I owe? (issuers compound daily on the average daily balance, so a real statement can differ by a few dollars)
Fixed-payment amortization — each month: interest = balance x APR/12; principal = payment - interest; closed form n = -ln(1 - rP/M) / ln(1+r). Verified 2026-07-31 against CFPB — How does paying down a mortgage work?

Frequently asked questions

How is credit card interest actually calculated?

Most issuers divide your APR by 365 to get a daily periodic rate, then apply it each day to your average daily balance, adding the result at the end of the cycle — that is the CFPB's description of standard practice. Calculators, including this site's, use the monthly equivalent (balance × APR ÷ 12) because it produces the same month counts and a far more readable schedule; the difference over one cycle is cents, and we state it rather than hide it.

What is a grace period?

If you pay your statement balance in full by the due date, most cards charge no interest on purchases at all — that is the grace period. It is why "carrying a balance builds credit" is a costly myth: the card works exactly the same for your credit report whether or not you leave a balance to accrue interest.

Why did I get charged interest even though I paid?

Usually residual interest — the interest that accrued between your statement date and the day your payment posted. It appears on the following statement and surprises people who paid in full and expected a zero. Ask the issuer for a payoff figure good through a specific date if you want the balance to land at exactly zero.

Does paying twice a month reduce interest?

Slightly, and in your favor. Because interest is charged on the AVERAGE daily balance, money that arrives mid-cycle lowers that average for the remaining days. The effect is modest — days, not years — but it is real, and it costs nothing to pay the day you get paid instead of the day it is due.

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