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How credit card interest works: grace periods and daily maths
The grace period's condition, APR chopped into daily pieces, penalty rates, where payments go, and why minimums are a trap.
Credit card interest feels arbitrary until you see the machinery: an annual rate, chopped into daily pieces, compounded on a balance that changes every day. Once the mechanics are visible, every "how did I get charged that?" question has an answer — and the CFPB documents the whole machine.
The grace period: the free month that is conditional
Most cards give purchases a grace period — the window between the end of a billing cycle and the payment due date — and if you pay the statement balance in full by the due date, purchases cost you nothing. The condition matters: if you were already carrying a balance, the grace period on new purchases is generally gone, and interest starts accruing immediately. Cash advances and convenience cheques typically have no grace period at all — interest starts the day you take the money, usually at a higher rate.
The daily machine
Card interest is usually calculated daily, not monthly. The issuer divides your APR by 365 to get the daily periodic rate, applies it to that day's balance, and adds the result back into the balance — interest compounding on interest, every day. Statements then aggregate this, typically via the average daily balance method. A 20% APR becomes about 0.0548% a day, which sounds trivial until it runs on a four-figure balance for a year. The practical consequence, as the CFPB notes: when there is no grace period, paying earlier in the month costs you less, because each day of balance is a day of interest.
Not one rate: the APR map
A single card can carry several APRs at once — purchases, cash advances, balance transfers, and a penalty APR that can be triggered on all balances if you are more than 60 days late. Your statement must show each category and the balance sitting under each rate, and it is worth reading once: the cash-advance number is usually the ugliest figure on the page.
Where your payment goes
The rules here are consumer-protective and worth knowing: if you pay more than the minimum, the issuer must generally apply the excess to the highest-rate balance first (the CARD Act's allocation rule). The minimum portion, though, can be applied however the issuer chooses — which is why paying only the minimum on a multi-rate card is the slowest possible way out of debt.
The minimum-payment trap
Minimum payments are typically a small percentage of the balance, so they shrink as the balance shrinks — which sounds gentle and is actually brutal: a modest balance at a typical APR paid at typical minimums takes years and can cost more in interest than the original purchases. This is precisely the arithmetic the desk's credit card payoff calculator runs honestly: fixed payment versus minimum payment, months and total interest side by side, working shown. The exit is never mysterious — a fixed payment, applied consistently, plus the avalanche logic from the debt-payoff guide — but seeing the two curves is what makes the trap visible.
Sources and further reading
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