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Credit Card Interest Calculator Guide
This shows what your balance is actually costing — per day, per cycle and per year — and how much of a payment disappears into interest before touching the debt.
Daily periodic rate = APR ÷ 365
Interest = average daily balance × daily rate × days in cycle
$5,000 at 20% APR over 30 days = 5,000 × 0.000548 × 30 = $82.19.
It's Daily, Not Monthly
Most people picture card interest as a monthly charge. It isn't. Interest is calculated on your balance every single day, and in most cases yesterday's interest joins the balance that today's interest is calculated on.
That's why the APR you were quoted isn't the rate you actually pay. Compounded daily, a 20% APR works out closer to 22.1% a year in practice. The gap widens as the rate rises.
The Grace Period Is the Whole Game
Pay your statement balance in full every month and you typically pay zero interest, regardless of your APR. The rate is irrelevant until you carry a balance.
Carry one, and the grace period usually disappears — meaning new purchases start accruing interest from the day you make them, with no interest-free window at all. That's the part nobody reads. You didn't just start paying interest on the old balance; you lost the free ride on everything new. Clearing the balance fully for a month or two normally restores it.
Average Daily Balance, Not Closing Balance
Card issuers usually charge on the average daily balance across the cycle, not the balance on the last day. Paying $2,000 off on day 28 of a 30-day cycle barely dents the interest — the balance was high for 27 days.
The practical consequence: paying earlier in the cycle costs less than paying the same amount later. Same money, less interest, purely from timing.
Cash Advances Are a Different Product
They usually carry a higher APR, charge a fee upfront, and have no grace period at all — interest starts the moment you take the money, even if you pay in full. Treating a cash advance like a purchase is one of the more expensive misunderstandings in personal finance.
Payments Are Applied Where It Suits the Issuer
If you have balances at different rates — a 0% transfer and a 24% purchase balance — minimum payments generally go to the lowest rate first. Anything above the minimum typically goes to the highest. So paying only the minimum on a mixed-rate card leaves the expensive balance growing untouched.
Related: credit card payoff calculator, minimum payment calculator, debt avalanche calculator.
These results are estimates for education and planning, not financial advice. Actual returns, rates, and terms vary — check with a qualified professional before making decisions.
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