Almost everything people believe about credit card interest is a month too coarse. It is charged daily, it compounds daily, and one common decision silently changes the rules for everything you buy afterwards.
Your APR is divided by 365 into a daily periodic rate, applied to your balance every day, and it compounds daily. Pay the statement balance in full and the grace period means purchases cost nothing extra. Pay anything less and you lose it — after which new purchases start accruing interest the day you make them, which is the part that surprises people.
The daily mechanics
Step 1 — the daily rate. Your APR divided by 365.
24.99% APR ÷ 365 = 0.0685% per dayStep 2 — the average daily balance. The issuer records your balance each day of the billing cycle, adds those figures up, and divides by the number of days. Paying early in the cycle lowers this average; paying on the due date does not.
Step 3 — apply and compound. The daily rate is charged against that balance each day, and yesterday's interest is part of today's balance.
That compounding is why the true annual cost of carrying a balance is somewhat higher than the stated APR — the APR is the nominal rate, not the effective one. Same distinction as APR vs APY.
The grace period, and how it disappears
This is the expensive part.
The grace period is the window between your statement closing and the payment due date. If you pay the statement balance in full, purchases in that cycle cost you no interest at all. That is why someone who always pays in full can carry a 29% APR card and never pay a cent of interest.
Now the part that is not obvious. Pay less than the full statement balance and the grace period switches off. After that, on most cards:
- The remaining balance accrues interest, as expected
- New purchases accrue interest from the transaction date — no interest-free window at all
- The grace period does not return until you pay in full, usually for one or two consecutive cycles
So the real cost of paying $400 of a $500 bill is not interest on $100. It is interest on $100 plus immediate interest on everything you buy next month.
US law requires that any payment above the minimum be applied to your highest-APR balance first. That is genuinely in your favour and worth using: if you are carrying a 0% transfer balance alongside purchases at 22%, paying more than the minimum attacks the 22% money.
The minimum payment is the exception. Issuers may apply it however they choose, and typically apply it to the lowest-rate balance. Paying only the minimum therefore leaves the expensive balance almost untouched.
Cash advances are a different product
They share a card and nothing else:
- No grace period. Interest starts the day you withdraw.
- A higher APR than purchases, usually by several points.
- An upfront fee, commonly a percentage of the amount with a minimum.
Cash-like transactions can count too — some money transfers, gambling and currency purchases. Worth checking your card's terms before assuming a transaction is a purchase.
What actually reduces the cost
Pay in full, every month. The only way to make the APR irrelevant. Below that, everything else is damage control.
Pay early in the cycle. Because the calculation uses the average daily balance, a payment on day 5 reduces more days of interest than the same payment on day 25.
Pay more than the minimum, so the surplus hits the highest-rate balance.
Ask for a lower APR. Unglamorous, free, and issuers do sometimes agree — particularly for long-standing accounts in good order.
Deal with the highest APR first when juggling several cards. That is the avalanche method — see how to pay off debt faster.
What does not help
Carrying a balance to build credit. A persistent myth. Utilisation and payment history are what score; paying interest adds nothing. See what actually moves your credit score.
Paying the minimum on time and feeling settled. On time protects your credit record. It does very little to the balance.
Closing a paid-off card reflexively. It removes available credit, which raises your utilisation ratio and can lower your score.
This is general information, not financial advice — see our disclaimer.
Frequently asked questions
Is credit card interest charged monthly?
No, it is calculated daily. The APR is divided by 365 to give a daily periodic rate, applied to your balance each day, and the interest itself compounds daily. That is why the annual cost exceeds a simple percentage of the balance.
What is the grace period?
The window between your statement closing and the due date, during which new purchases carry no interest if you pay the statement balance in full. It only exists while you pay in full every month.
What happens if I pay only part of my bill?
You lose the grace period. Interest applies to the remaining balance and, on most cards, to new purchases from the day you make them — not from the next statement. Getting it back usually needs one or two full payments.
Which balance does my payment reduce first?
By law in the US, any amount above the minimum payment must go to the highest-APR balance first. The minimum payment itself, however, is applied at the issuer's discretion, usually to the cheapest balance.
Do cash advances work the same way?
No, and they are worse in three ways: usually no grace period at all, a higher APR, and an upfront fee. Interest starts the moment you take the cash.
Sources
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