What the advertisement has to tell you. 12 CFR 1026.16(g)(4) requires that where a promotional rate may apply, the advertisement state clearly and conspicuously when the promotional rate will end and the annual percentage rate that will apply after the promotional period, and in a written or electronic advertisement that information must also appear "in a prominent location closely proximate to the first listing of the promotional rate." If the go-to rate cannot be determined in advance because it depends on the applicant's creditworthiness, the advertisement must disclose the specific rates or the range of rates that might apply. So the end date and the rate after it are not things an issuer may decide later.
The six-month floor is an exception, not a prohibition, and the distinction changes what you should expect. The general rule at 12 CFR 1026.55(a) is that a card issuer must not increase an annual percentage rate on a credit card account at all. The temporary rate exception at 1026.55(b)(1) permits an increase "upon the expiration of a specified period of six months or longer," but only where, before the period began, the issuer "disclosed in writing to the consumer, in a clear and conspicuous manner, the length of the period and the annual percentage rate, fee, or charge that would apply after expiration of the period." A promotion advertised as three months cannot end in a rate increase on that route. What the six months does not mean is that the rate is untouchable for six months, because other exceptions in the same paragraph remain available. The one that matters most is the delinquency exception at 1026.55(b)(4), which lets an issuer raise the rate where a required minimum payment is not received within 60 days of its due date, and which must then cease to apply once the issuer receives six consecutive required minimum payments on or before the due date. Read together, the honest statement is that a promotion has to be at least six months long, and that losing it early is generally a consequence of your own delinquency rather than of the issuer's discretion.
What the promotion covers is not obvious from the headline. 12 CFR 1026.60(b)(1) requires the terms table to disclose each periodic rate that may be used to compute the finance charge on purchases, on a cash advance, and on a balance transfer, separately. A card can therefore run 0% on purchases while charging its standard rate on transfers, or the reverse, and the promotional periods for the two need not be the same length. The category of transaction the promotion applies to is the first thing to read in the table, not the number in the advertisement.
When the promotion expires, 1026.55(b)(1)(ii)(A) protects balances that predate it: the issuer must not apply a rate to transactions occurring before the period that exceeds the rate which applied to them beforehand. That matters where a promotion was added to an existing account, and the balance transfer page covers it for a transferred balance.
The instrument most often confused with this one is deferred interest, and the confusion is the expensive kind. 12 CFR 1026.16(h)(2) defines deferred interest as "finance charges, accrued on balances or transactions, that a consumer is not obligated to pay or that will be waived or refunded to a consumer if those balances or transactions are paid in full by a specified date," and adds that it "does not include any finance charges the consumer avoids paying in connection with any recurring grace period." So a genuine 0% promotion charges nothing during the window and begins charging on whatever is left afterward. A deferred-interest offer is charging the whole time and merely withholding the bill.
Regulation Z makes the advertising tell you which one you are looking at. Under 1026.16(h)(3), where "no interest" or a similar term is used, "the term 'if paid in full' must also be stated in a clear and conspicuous manner preceding the disclosure of the deferred interest period," and in a written advertisement it must appear in immediate proximity to each statement of "no interest," "no payments," "deferred interest," "same as cash," or similar term. Under (h)(4) the advertisement must also state that interest will be charged from the date the consumer became obligated for the balance if it is not paid in full within the period, and where applicable, if the account goes into default before the period ends. The four words "if paid in full" are the whole difference, and they are required to be there.
The Bureau's account of the consequences is worth having exactly: miss the deadline, "or if you are more than 60 days late in making a minimum payment," and you owe "all of the interest back to the original date of the charge." Payments above the minimum go to your highest-rate balance until "the last two billing cycles in your deferred interest period," when the excess is directed to the deferred balance instead. Deferred interest is overwhelmingly a retail and private-label product rather than a general-purpose card feature: the Bureau's December 2025 report to Congress records more than $70 billion of purchases on deferred-interest plans in 2024, and notes that the average retail rate on private label cards exceeded 31 percent at the end of that year, which is the rate that lands retroactively when the deadline is missed.
