Credit cards, mechanically

The credit card is the most widely used consumer credit product in the United States and one of the most precisely regulated, with the disclosure framework set by the Truth in Lending Act and substantially revised by the CARD Act of 2009. The product combines a payment instrument (the card itself, operating on the four-party network model described in what happens when you swipe a card) with a revolving credit line (an unsecured loan the cardholder can draw down through purchases). The economics of the product are built around a deliberate ambiguity: it is a payment tool for the transactor who pays in full, and a credit instrument for the revolver who carries a balance, and the issuer earns from both.

This article describes the mechanics of revolving credit: the statement cycle, the grace period, how interest is actually computed on a carried balance, the minimum-payment formula, and the disclosures that surround all of it. For the underlying payment-network mechanics, see what happens when you swipe a card; for the regulatory framework, see the Truth in Lending Act.

The statement cycle

A credit-card account runs on a monthly billing cycle of approximately 30 days. At the end of each cycle, the issuer produces a statement that captures all transactions posted during the cycle, applies any payments and credits, computes any interest charges, and sets a minimum payment due and a payment due date. Regulation Z requires the payment due date to be the same calendar day each month and at least 21 days after the statement closing date (for cards that offer a grace period).

The statement also discloses the new balance, the previous balance, fees imposed, interest charges, the APR(s) applicable to different balance types (purchases, cash advances, balance transfers, penalty), the year-to-date interest and fees, and the minimum-payment warning showing how long it would take to pay off the new balance making only minimum payments. The minimum-payment warning is a 2009 CARD Act requirement and is among the most consequential disclosures in U.S. consumer-credit law: it makes the cost of carrying a balance directly visible on every statement.

The grace period

The grace period is the interval between the statement closing date and the payment due date during which no interest accrues on new purchases, provided the prior statement balance was paid in full by the prior due date. The grace period is not a federal mandate, but a market convention universally offered on consumer general-purpose cards; the convention is reinforced by Regulation Z provisions that limit how issuers can treat balances on cards that offer a grace period.

The grace period works as follows. On Day 0 of the cycle, the cardholder has a $0 balance. During the cycle (Day 1 through Day 30), the cardholder makes purchases totaling $1,000. On Day 30, the cycle closes with a statement balance of $1,000. The cardholder has 21 to 25 days from the closing date until the payment due date. If the cardholder pays the full $1,000 by the due date, no interest accrues. The cardholder has used the issuer's money interest-free for an average of roughly 25 days (15 days for the purchase made mid-cycle plus the grace-period extension).

The grace period applies to new purchases only when the prior balance was paid in full. If the cardholder carries any balance from the prior cycle, new purchases begin accruing interest from the date of the transaction (or the next business day) rather than enjoying the grace period. This is why "paying mostly off" a credit card produces a much worse outcome than paying it off entirely: the partial-payment cardholder loses the grace period on all new purchases and accrues interest from the date of each transaction.

How interest is computed

For a cardholder carrying a balance, the interest computation follows a standard method specified in the account agreement and disclosed under Regulation Z. The most common method at U.S. credit-card issuers is the average daily balance method:

1. For each day in the billing cycle, the issuer calculates the balance owed.
2. The sum of the daily balances is divided by the number of days in the cycle to produce the average daily balance.
3. The average daily balance is multiplied by the daily periodic rate (the APR divided by 365) and by the number of days in the cycle to produce the interest charge.

The "two-cycle billing" method, which used a two-month average rather than a one-month average and was historically used by some issuers, was effectively eliminated by the CARD Act. The average daily balance method is now standard.

For a cardholder who carries a $5,000 balance for a full cycle at a 20% APR, the daily periodic rate is 20% ÷ 365 = 0.0548%. Applied to the $5,000 balance over 30 days, the interest charge is approximately $82.20 for the cycle, or about $986 per year. Note that the actual annual cost is meaningfully higher than the 20% APR because of the compounding effect; the effective annual rate on a continuously carried balance compounded daily at 20% APR is approximately 22.13%. This compounding gap — present in every U.S. consumer credit-card product — is one reason the APR understates the true cost of carrying a balance for borrowers planning to do so for a full year. See APR versus APY.

The minimum payment

The minimum payment is the smallest amount the cardholder must pay each cycle to keep the account current. It is calculated under a formula specified in the account agreement; typical formulas include:

• The greater of $25, $35, or 1% to 2% of the new balance plus interest and fees for the cycle.
• For accounts with very low balances, a flat minimum (e.g., $15 if the balance is at least $15).

The structure is designed to require the cardholder to amortize the balance very slowly. A $5,000 balance at 20% APR with a 2% minimum payment produces an initial minimum of approximately $182 (about $83 interest plus $100 principal). At that rate of principal repayment, the balance takes approximately 27 years to pay off, with total interest paid of more than $7,500 — more than the original $5,000 principal. The minimum-payment warning required by the CARD Act on every periodic statement makes this explicit; the cardholder sees, on the same statement that asks for the minimum, that paying only the minimum will take decades and cost multiples of the original purchase amount.

Other fees and the CARD Act restrictions

Credit cards historically carried a wide variety of fees: annual fees, late fees, over-limit fees, foreign-transaction fees, balance-transfer fees, cash-advance fees, returned-payment fees, "convenience" fees. The CARD Act of 2009 substantially restricted several of these:

• Late fees were capped at a "reasonable" amount, with the CFPB setting safe-harbor caps that are updated periodically (the CFPB issued a final rule in 2024 substantially lowering the safe-harbor cap; the rule has been subject to litigation — verify current status).
• Over-limit fees can only be charged if the cardholder has affirmatively opted into over-limit transactions; without opt-in, the issuer must decline transactions that would exceed the limit.
• Penalty APRs (a higher rate triggered by late payment) can only be applied to existing balances under defined conditions, with reduction back to the original rate after six consecutive months of on-time payments.
• Annual fees can be charged but must be disclosed clearly in the Schumer box on every solicitation.
• Inactivity fees and certain other "deactivation" fees were prohibited.

Several recurring fees remain: balance-transfer fees (typically 3% to 5% of the transferred balance, charged upfront), cash-advance fees (typically 3% to 5% of the advance amount, plus a much higher cash-advance APR with no grace period), and foreign-transaction fees (typically 1% to 3% of foreign-currency purchases, though many premium and travel-focused cards have eliminated this fee).

Rewards economics

The rewards programs attached to many credit cards — cashback, travel points, transferable miles — are funded by interchange revenue plus the spread the issuer earns on revolving balances. Higher-rewards cards typically carry higher interchange rates (set by the network's rate schedule, with premium-card categories carrying higher merchant fees), and merchants ultimately bear that cost.

The CFPB has documented that rewards-card revenue dynamics produce a regressive transfer: merchants set prices to recover their card-acceptance costs, so cash and debit-card users effectively subsidize the rewards earned by credit-card users (particularly users of premium cards with the highest interchange). The dynamic has been the subject of academic and regulatory debate; the regulatory response so far has been limited to the debit-card interchange cap (which does not affect credit cards) and to periodic CFPB attention to rewards-program transparency.

Limits and uncertainty

The CARD Act framework has been stable since 2009; the late-fee safe-harbor rule and related amendments continue to evolve. The interchange-cap debate that has produced regulation on debit cards has periodically been raised for credit cards (the Credit Card Competition Act being the most prominent recent legislative vehicle); whether and how credit-card interchange will be regulated is uncertain. The basic mechanics of revolving credit — statement cycles, grace periods, daily-periodic-rate interest computation, and the minimum-payment formula — have been stable for decades and are unlikely to change in the foreseeable future. The specific dollar amounts and fee structures move with regulation and competitive dynamics.