How Credit Card Interest Works: APR, Grace Periods and Daily Balances

Independent guide - not affiliated with or endorsed by any card issuer or card network. Rates and fees differ by card and change over time; your cardholder agreement and the issuer's official pricing page are the final word.

Credit card interest is charged when you carry a balance past the due date. Most issuers turn your APR into a small daily rate, apply it to your balance each day of the billing cycle, and add the total to your next statement. Pay the full statement balance by the due date and, on most cards, new purchases cost you no interest at all.

That short answer hides a few moving parts that catch people out: the grace period, the average daily balance, separate rates for different kinds of transactions, and the "trailing" interest that can appear even after you think you have paid a card off. This guide walks through each one using the rules published by the US Consumer Financial Protection Bureau (CFPB). It explains the mechanics, not any particular card's pricing. For the rates on a specific card, read its pricing table on the issuer's website.

APR, daily periodic rate and why interest is calculated daily

Every card states its interest as an annual percentage rate, or APR. Issuers rarely charge it once a year, though. The CFPB explains that many card companies calculate interest daily, based on your average daily balance. To do that they divide the APR into a daily periodic rate. Your cardholder agreement says whether the issuer divides by 365 or by 360 days. Either way, the daily rate is a very small percentage applied many times.

The calculation works like this:

1. The issuer records your balance at the end of each day in the billing cycle.
2. It adds those daily balances together and divides by the number of days in the cycle. The result is your average daily balance.
3. It multiplies the average daily balance by the daily periodic rate and by the number of days in the cycle.

Some issuers compound daily, meaning each day's interest is added to the balance that earns interest the next day. The method your card uses is in the agreement under a heading such as "How we calculate your balance". Because interest builds day by day, the CFPB notes that paying down a balance earlier in the cycle reduces the interest you owe when you do not have a grace period.

The grace period: how to pay no interest on purchases

A grace period is the time between the end of a billing cycle and the payment due date. The CFPB says card issuers are not required to offer one, but most do. If your card has a grace period and you pay the full statement balance by the due date, you are generally not charged interest on purchases.

Federal rules also control the timing. Under Regulation Z, the issuer must mail or deliver your statement at least 21 days before the payment due date, so you have time to pay in full and keep the grace period.

The catch is that the grace period is fragile:

• Carry any balance and you can lose it. The CFPB explains that if you carry a balance from one month, you can lose the grace period on new purchases. Interest then starts on new purchases from the day you make them.
• Getting it back can take more than one month. If you pay in full some months and not others, the CFPB says you may lose the grace period for the month you do not pay in full and for the month after.
• It usually covers purchases only. Cash advances and balance transfers normally start accruing interest straight away.

Our guide on how to avoid credit card interest and fees turns this into a simple monthly routine.

One card, several APRs

Most cards have more than one rate. The CFPB notes that card companies often charge one rate for purchases and different rates for cash advances or other transactions. A typical agreement can list:

Your monthly statement must show each rate and the balance it applies to. If you are using a promotional offer, our guide to how a balance transfer works covers the fine print, and our 0% intro APR rankings compare current offers.

How your payments are applied

When balances carry different rates, the order in which your payment is applied matters. The CFPB explains that when you pay more than the minimum, the issuer generally must apply the extra amount to the balance with the highest rate first, then to the others in descending order of rate. The issuer generally decides how the minimum payment itself is applied. In practice, paying only the minimum can leave the most expensive balance growing while the cheaper one shrinks.

Why you might still be charged interest after paying in full

This is one of the most common surprises. If you were carrying a balance, interest keeps accruing every day between the date your statement was produced and the day your payment arrives. That interest was not on the statement you paid, so it shows up on the next one, even though you paid the "full" amount you were shown. It is often called residual or trailing interest.

Two steps usually stop it:

• Pay the full statement balance for two cycles in a row, so the grace period is restored and no new daily interest builds.
• If you want to close out the balance completely, ask the issuer for a payoff amount for a specific date, or check the current balance in the app on the day you pay.

If a charge still looks wrong, you can dispute it as a billing error. Our guide on how to dispute a credit card charge explains the process.

Why your interest charge can look so high

Card APRs are usually much higher than rates on secured loans, and daily accrual on a large balance adds up quickly. Other common reasons for a bigger-than-expected interest line are:

• A promotional rate ended and the standard rate now applies.
• A cash advance, which typically has its own higher rate and no grace period. See our explainer on credit card cash advances.
• A variable APR rose because the index it is tied to rose. Your agreement says which index is used.
• A penalty APR was applied after a late payment.

For most rate increases, Regulation Z requires the issuer to give you advance written notice and, in many cases, the right to reject the change on existing balances. The CFPB's page on whether an issuer can change your terms explains the rules.

The minimum payment warning on your statement

Your monthly statement must include a minimum payment warning. It shows how long it would take to pay off the balance and roughly how much you would pay in total if you paid only the minimum each month, alongside the monthly payment needed to clear it in about three years. It is worth reading at least once: it is the clearest picture of what carrying a balance actually costs on your card at your rate.

A simple way to estimate your own interest

You do not need a calculator app to get a rough figure. Take your card's purchase APR from the statement, divide it by 365 to get the daily rate, then multiply by your average balance and by the number of days in the cycle. The result will be close to the interest line on your statement. If it is not, check whether part of your balance sits at a different rate, such as a cash advance or an expired promotion.

The habits that keep interest at zero

• Set autopay for the full statement balance, not the minimum.
• If you cannot pay in full, pay as much as you can as early in the cycle as you can.
• Avoid cash advances, which skip the grace period.
• Track promotional end dates in your calendar.
• Read the rate section of every statement for changes.

If you are choosing a new card, the purchase APR matters most if you might carry a balance; if you always pay in full, rewards and fees matter more. Our five-question tie-breaker shows how to weigh them.

Sources

• CFPB: How does my credit card company calculate the amount of interest I owe?
• CFPB: What is a grace period for a credit card?
• Regulation Z, 12 CFR 1026.7: Periodic statement
• CFPB: Can my credit card company change the terms of my account?
• CFPB: Credit cards key terms