How Does Credit Card Interest Work? The Rules Explained

Quick Answer: Credit card interest is charged on balances you carry past the due date, at a yearly rate called the APR that issuers usually apply daily. Most cards give you a grace period on purchases: pay the full statement balance by the due date and no purchase interest is charged. Carry any balance, or take a cash advance, and interest builds day by day and compounds.

Credit card interest works on a simple trade: the issuer lends you money between your purchase and your payment, and only charges for that loan if you do not repay the whole statement balance by the due date. When it does charge, it converts the APR into a daily rate, applies it to your balance each day, and adds the result to what you owe.

The vocabulary you need first

Before the rules make sense, five terms need to be clear. Each of them appears, in one form or another, on your monthly statement.

• APR (annual percentage rate). The CFPB describes a card's interest rate as the price you pay for borrowing money, expressed as a yearly rate called the APR. A card usually has several: one for purchases, one for cash advances, one for balance transfers, sometimes a penalty rate.
• Daily periodic rate. The APR divided across the days of the year. This is what is actually applied to your balance each day.
• Billing cycle. The roughly monthly window of transactions that one statement covers. It ends on the closing date.
• Grace period. The CFPB defines it as the period between the end of a billing cycle and the date your payment is due.
• Balance subject to interest. The amount the rate is applied to, most often an average of your daily balances across the cycle.

If you want to see where those numbers sit on your own bill, the guide to finding your credit card interest rate points to the exact section.

When does interest start?

This is the part people most often get wrong, because the answer depends on the type of transaction and on what happened last month.

Chase's education pages state this plainly: grace periods do not apply to cash advances or balance transfers, and interest on a cash advance taken at an ATM begins accruing the same day. The CFPB adds that grace periods generally apply only to new purchases, and only if you are not already carrying a balance.

Do credit cards charge interest if you pay on time?

It depends what "pay on time" means. There are three versions, and they produce very different outcomes:

1. You pay the full statement balance by the due date. No purchase interest for that cycle. This is the whole point of the grace period, and the CFPB says on most cards you can avoid paying interest on purchases this way.
2. You pay the minimum by the due date. No late fee and no damage to your payment history, but the unpaid part of the statement balance is charged interest. On most cards you also lose the grace period, so new purchases start accruing from the day they post.
3. You pay something in between. The same as paying the minimum, just on a smaller leftover balance.

The CFPB notes that card companies are not required to offer a grace period at all, although most do on regular purchases. If yours has one, the length is printed on the statement and in the agreement. Statements must reach you at least 21 days before the due date, which is why the grace period on most cards lands at three weeks or slightly more.

Is credit card interest simple or compound?

Compound. This is the answer to "are credit cards compounded monthly" as well: they usually compound more often than that.

Here is the mechanism the CFPB describes. Many issuers calculate interest daily, based on your average daily balance. That daily balance includes interest that has already been charged in previous cycles. So once interest posts to your account and you do not pay it off, it becomes part of the balance that tomorrow's interest is calculated on. Chase's explanation of when cards charge interest says the same thing more bluntly: interest compounds daily, using the APR divided by 365.

What that means practically:

• Carrying a balance costs slightly more than the headline APR suggests, because interest is charged on interest.
• Paying earlier in the cycle reduces your average daily balance, so the CFPB points out that paying sooner reduces the total interest even if you cannot pay in full.
• A balance you leave alone keeps growing even if you never swipe the card again.

The arithmetic of turning an APR and a balance into a dollar figure is a separate question, covered step by step in how to calculate credit card interest.

How you lose and regain the grace period

Losing the grace period is the single most expensive thing that can happen on a card you normally pay in full, because it changes the treatment of every new purchase.

You typically lose it by leaving any part of a statement balance unpaid after the due date. The CFPB warns that missing a full payment even once can cost you the grace period for that cycle and possibly the next. Chase says it may reinstate a grace period if you pay your balance in full for two consecutive billing cycles; other issuers set their own rule, which will be in the cardholder agreement.

While the grace period is gone, two effects stack. New purchases start accruing interest when they post. And when you do finally pay the statement balance, interest that accrued between the closing date and your payment date (often called trailing or residual interest) shows up on the following bill. Paying the next statement in full clears it and, on most cards, restores the interest-free treatment.

How payments are applied when you carry more than one balance

If your card has balances at different rates, say purchases at one APR and a promotional transfer at another, the order in which your payment is applied changes how much interest you pay. The CFPB explains the rule: anything you pay above the minimum must go first to the balance with the highest interest rate, then to the next highest. How the minimum payment itself is applied is set by the card's terms, so check the agreement if you rely on a promotional rate.

Can the rate itself change?

Yes, within limits set by the CARD Act and written into Regulation Z, section 1026.55:

• Variable APRs move with a public index, typically the prime rate. When the index rises, your rate can rise with it.
• Promotional rates can end and revert to the standard rate, but only after a promotional period of at least six months that was disclosed in writing.
• Penalty APRs can be applied to existing balances only when a minimum payment is more than 60 days late, and the issuer must reduce the rate again after six consecutive on-time minimum payments.
• Other increases generally require written notice at least 45 days before they take effect, and in the first year after opening an account increases are restricted to these exceptions.

Watching interest across your statements

Every statement lists interest charged for the period and year-to-date, so the quickest check is the interest section of the latest bill. If you want to see a year of interest and fees side by side, converting the PDFs gives you rows you can filter. QuickBankConvert turns a card statement into a spreadsheet, one transaction per row with interest and fee lines included as they are printed, via the convert page; your issuer's own CSV download may already cover recent months. The statement balance vs current balance guide explains which of the two numbers the grace period depends on.

Related guides

• How to read a credit card statement
• How to calculate credit card interest
• How to find your credit card interest rate
• CFPB: How does my credit card company calculate the amount of interest I owe?
• CFPB: Regulation Z, section 1026.55, limits on increasing rates