A purchase interest charge on credit cards is the finance charge your issuer adds to the cost of goods and services you bought with the card whenever you carry a balance past the payment due date. Most cards express this as a variable Annual Percentage Rate, which as of early 2026 averages roughly 19.5% nationally. It applies only to purchases. Cash advances and balance transfers carry their own rates, usually higher, and follow separate rules.
How Your Purchase APR Is Set
Nearly all credit cards use a variable purchase APR that moves with an external benchmark: the prime rate, which tracks the Federal Reserve’s federal funds rate and typically sits about three percentage points above it. Your APR equals the prime rate plus a fixed margin the issuer assigned when you were approved. If prime is 7.5% and your margin is 14%, your purchase APR is 21.5%.1Consumer Financial Protection Bureau. Credit Card Interest Rate Margins at All-Time High
When the Fed moves its benchmark, your APR moves with it automatically, and the issuer doesn’t have to send a fresh notice because the variable mechanism was disclosed at account opening. What the issuer cannot change without warning is the margin itself. Federal law requires 45 days’ advance notice before raising that fixed component of your rate.2Office of the Law Revision Counsel. 15 USC 1666i-1 – Limits on Interest Rate, Fee, and Finance Charge Increases Applicable to Outstanding Balances
How the Charge Is Calculated
Interest isn’t tallied once a month on your ending balance. Issuers calculate it daily and compound it, so yesterday’s interest becomes part of today’s balance. The math starts by dividing your APR by 365 (some issuers use 360) to get a daily periodic rate.3Consumer Financial Protection Bureau. What Is a Daily Periodic Rate on a Credit Card? A 19.99% APR works out to about 0.0548% a day.
That daily rate is applied to your average daily balance for the billing cycle. Your issuer records your balance at the close of each day, adds those figures together, and divides by the number of days in the cycle (usually 28 to 31). Say you start a 30-day cycle owing $2,000 and charge another $1,000 on day 16. Your average daily balance is $2,500. Multiply $2,500 by 0.0548% and then by 30 days, and you owe roughly $41 in interest for that cycle.
Because the process compounds daily, you pay interest on interest. Over several months of carrying a balance, that gap widens quickly. Your monthly statement has to itemize the finance charge, the APR applied, and each balance category so you can trace how the number was reached.4eCFR. 12 CFR 1026.7 – Periodic Statement
The Grace Period Is How You Avoid the Charge
The grace period is the window between your statement closing date and your payment due date. Pay the full statement balance by the due date, and no purchase interest is charged. Federal law does not require issuers to offer a grace period, but most do.5Consumer Financial Protection Bureau. Comment for 1026.54 – Limitations on the Imposition of Finance Charges When one is offered, Regulation Z requires the statement to reach you at least 21 days before the grace period ends.6eCFR. 12 CFR 1026.5 – General Disclosure Requirements
The rule is all or nothing. Pay $950 on a $1,000 balance and you don’t just get charged interest on the remaining $50. You lose the grace period entirely, and interest starts running on purchases from the date each one was made.7Consumer Financial Partners Bureau. What Is a Grace Period for a Credit Card? New purchases in the following cycle also start accruing interest from their transaction date. To get the grace period back, most issuers require you to pay in full for one or two consecutive cycles, with interest still building during the payoff.
The grace period covers purchases only. Cash advances and convenience checks start accruing interest immediately from the transaction date. Balance transfers follow the terms in their promotional offer.
Residual Interest After You Pay in Full
Even after paying the statement balance in full, you may see a small interest charge on the next statement. That’s residual interest. Your statement balance reflects what you owed on the closing date, but interest keeps accruing between that date and the day your payment posts. Those extra days show up one cycle later.
If you’re clearing a balance you’ve carried for months, call your issuer and ask for the payoff amount including residual interest. It will be slightly higher than the statement balance but will zero out the account. Once the next cycle closes with a zero balance, the grace period kicks back in and residual charges stop.
Penalty APR
Miss a payment by more than 60 days and your issuer can impose a penalty APR, often close to 30% and frequently the highest rate they’re allowed to charge. Unlike an ordinary rate increase, a penalty APR can be applied to your existing balance, not just future purchases.2Office of the Law Revision Counsel. 15 USC 1666i-1 – Limits on Interest Rate, Fee, and Finance Charge Increases Applicable to Outstanding Balances
There is a way back. Make your minimum payments on time for six consecutive months after the penalty rate takes effect, and the issuer must restore your previous rate on the pre-existing balance.2Office of the Law Revision Counsel. 15 USC 1666i-1 – Limits on Interest Rate, Fee, and Finance Charge Increases Applicable to Outstanding Balances The penalty rate may still apply to new purchases, but the six-month clock gives you a concrete recovery path. The issuer must explain this right in the same notice announcing the increase.
How Payments Are Applied When You Carry Multiple Balances
Many accounts carry more than one balance type: purchases at 18%, a balance transfer at 0% for a promotional stretch, a cash advance at 26%. Federal law dictates how your payment is distributed, and it works in your favor. Any amount above the minimum must go first to the balance with the highest APR, then to the next highest.8eCFR. 12 CFR 1026.53 – Allocation of Payments Without that rule, an issuer could direct extra payments at a 0% promotional balance while a 26% cash advance kept compounding.
The minimum payment itself can still be allocated however the issuer chooses. Paying more than the minimum is what activates the highest-APR-first rule, which is why paying above the minimum matters most when you carry mixed balances.
One exception applies to deferred-interest promotions. During the last two billing cycles before that kind of promotion expires, excess payments must go to the deferred-interest balance first.8eCFR. 12 CFR 1026.53 – Allocation of Payments Deferred interest is not waived interest. If any of that balance remains when the promotion ends, the issuer can retroactively charge you all the interest that would have accrued from day one. The final-two-cycle allocation shift is meant to help you avoid that outcome.