How credit card interest really works
Last updated November 9, 2026
Credit card interest can feel mysterious, but the idea is simple. If you carry a balance past the due date, the card charges interest on it. Knowing how that charge is figured helps you avoid it or at least shrink it.
APR becomes a daily rate
Your card states an annual percentage rate, or APR. Most issuers divide it by 365 (some use 360) to get a daily rate, then apply that rate to your balance every day. A 20% APR works out to about 0.055% a day.
Interest adds to your balance
The interest charged each day is added to what you owe, so the next day's interest is calculated on a slightly larger amount. That is compounding. With daily compounding, a 20% APR costs about 22.1% over a full year if you carry the same balance the whole time.
The grace period
If you pay your full statement balance by the due date every month, most cards charge no interest on new purchases. That interest-free window is the grace period. Once you carry a balance, the grace period usually goes away until you pay the full balance again, so new purchases start collecting interest right away.
Other balances may have different rules
- Cash advances often start charging interest immediately, with no grace period, and may carry a fee.
- Balance transfers can come with a low introductory rate, but usually carry a transfer fee, often a few percent of the amount moved. Check what rate applies when the offer ends.
- Penalty rates can apply after missed payments. Read your card agreement.
A note about our calculator
The planner divides your APR by 12 and applies it once a month. Cards figure interest daily, so your real statement can differ by a small amount. The estimates are still good for comparing payoff orders.
Ways to pay less interest
- Pay the full statement balance when you can.
- When you cannot, pay as much as possible as early as possible in the cycle, because interest builds daily.
- Put extra money on the card with the highest APR first.
Run your own numbers
Enter your balances, rates and payments to see which debt to pay first and when you could be debt-free.
Related guides
- What APR means and how to use it
- Should you pay off your car loan early?
- Pay off student loans early or invest instead?
- Is a balance transfer worth it?
This article is for education only and is not financial, tax or legal advice. Examples use simplified monthly interest and assume on-time payments with no new borrowing.