The difference is what happens to the interest during the promotion. Under a true 0% APR, no interest exists, so there is nothing to bill later. Under deferred interest, interest is calculated from day one at the card’s ordinary retail APR and simply held back, and if any balance remains at the deadline the whole accrued amount is charged at once. Both are advertised with the words no interest. Only one of them means it.
A note on scope before the numbers: deferred interest is a purchase-financing product, typically found on store and retail cards, applied to a specific purchase. It is not a balance transfer product, and the mainstream US balance transfer promotions are true 0% offers. The calculator above models the retail deferred interest case on its own terms.
How a true 0% promotion works
Interest is waived on the promotional balance for the promotional period. Nothing accrues, so nothing accumulates in the background. When the period expires, whatever balance is still outstanding begins accruing at the disclosed go-to APR from that point forward. Paying off 90% of the balance leaves the remaining 10% exposed, and only that 10%. The cost of falling short is proportional to how far short.
How deferred interest works
The issuer calculates interest at the retail APR on the balance as it declines from the purchase date, and holds the total. Two outcomes are possible, and there is no middle one.
Pay the promotional balance in full by the deadline and the accrued interest is waived. Leave any amount outstanding, even a dollar, and the full accrued total is capitalised into the balance. The CFPB describes it plainly: the issuer calculates how much interest the consumer would owe at the account’s retail APR but does not immediately charge it, and a consumer who does not pay in full during the promotional period will generally have all the deferred interest capitalised at the promotion’s conclusion. The same report noted that the average retail APR on private label cards exceeded 31% at the end of 2024.
The charge is not interest on the leftover. It is interest on the entire balance over the entire promotional period, retrospectively.
Worked example: $2,000 at 29.99% over 12 months
A $2,000 purchase on a 12-month no-interest-if-paid-in-full promotion, at a 29.99% retail APR, with $150 paid each month.
Twelve payments of $150 total $1,800. That leaves $200 outstanding on the deadline. The deferred interest accrued across those twelve months on the declining balance comes to roughly $310 to $350 depending on when payments post within each cycle, so call it about $330. That amount is added to the balance. The $200 leftover becomes about $530, and it starts accruing at 29.99% going forward.
Now the same purchase cleared on time. Paying $2,000 divided by 12, which is $166.67 a month, brings the balance to zero on the deadline. The accrued interest is waived. Total cost: $2,000.
| $150 a month | $166.67 a month | |
|---|---|---|
| Paid over 12 months | $1,800 | $2,000 |
| Balance at the deadline | $200 | $0 |
| Deferred interest billed | about $330 | $0 |
| Owed after the deadline | about $530 | $0 |
The gap between the two payment plans is $16.67 a month. The gap in outcome is about $330 billed in a single statement plus a balance still running at nearly 30%. Under a true 0% offer, the $150-a-month plan would have left $200 owing and nothing more.
Why careful payers still get caught
Three mechanisms, none of them obvious from the advertising.
The promotional balance sits on a card that can hold other purchases at the full retail APR. Under 12 CFR 1026.53, above-minimum payments go to the highest-APR balance first, so those other purchases absorb the extra money while the promotional balance drains only at the minimum. In the last two billing cycles of the promotion, 1026.53(b)(1)(i) reverses this and sends above-minimum payments to the deferred balance first, which is a genuine protection but arrives late.
The minimum payment is not calibrated to clear the balance by the deadline. It is calibrated to the issuer’s ordinary minimum formula. Paying the minimum every month on time can still leave a balance on the deadline, and that is enough.
And the deadline is not the same as the statement due date, which is why 12 CFR 1026.7(b)(14) requires each periodic statement during the promotion to show the date by which the balance must be paid in full to avoid the finance charges.
The evidence that this is not a fringe problem is in the CFPB’s own findings: more than half of consumers who incurred deferred interest charges had paid more than the full promotional balance during the promotional period, and over a third had paid more than 150% of it. The CFPB has encouraged retailers to switch to true 0% promotions. The National Consumer Law Center’s 2015 analysis put average APRs on these cards around 24% and as high as 29.99%.
What the rules require in the advertising
Regulation Z at 12 CFR 1026.16(h) sets the framing. The deferred interest period must be stated clearly and conspicuously. If no interest language is used, the words if paid in full must accompany it. And the advertisement must carry a statement that interest will be charged from the date the consumer becomes obligated for the balance if it is not paid in full within the period, placed in a prominent location closely proximate to the first mention of the offer. Under the CARD Act and 1026.55(b)(1), deferred interest promotions shorter than six months are effectively prohibited.
The practical reading for anyone standing at a checkout: the offer is genuinely worth taking if the balance divided by the number of months is affordable and will actually be paid, because the interest cost is then zero. It is a poor deal at any payment level that leaves a balance behind, because the penalty is not proportional. The single test worth applying is whether the full amount can be cleared by the deadline, and the calculator above computes the required monthly payment and the retroactive charge for a given plan.
For the different question of what happens when a genuine 0% period expires on a transferred balance, see what happens after the 0% ends, and for how minimum payments behave on promotional balances, balance transfer minimum payments.
Common questions
Does 0% APR mean no interest?
With a true 0% promotional APR, yes, for the promotional balance during the promotional period. No interest accrues, so nothing can be billed retroactively. When the period ends, only whatever balance is still outstanding starts accruing at the go-to rate, going forward. Deferred interest offers are different and should never be described as 0% APR.
What is deferred interest on a store card?
Interest is calculated at the card's ordinary retail APR from the purchase date, but it is held back rather than billed. If the promotional balance is paid in full by the deadline, that accrued interest is waived. If any part of the balance remains, the whole accrued amount is added to the balance at once. The CFPB describes this as interest being calculated but not immediately charged.
How much retroactive interest can be charged?
It is the interest that accrued on the balance as it declined across the whole promotional period, not interest on the small amount left over. On a $2,000 purchase at 29.99% paid down over twelve months, that is roughly $310 to $350 depending on how payments post. Leaving $1 unpaid triggers the same charge as leaving $500 unpaid.
Why do people who pay a lot still get charged?
Payments do not always land where the cardholder assumes. Above-minimum payments go to the highest-APR balance first, so other purchases on the same card can absorb them while the promotional balance sits there. The CFPB found in 2017 that more than half of consumers who incurred deferred interest charges had paid more than the full promotional balance during the promotional period, and over a third had paid more than 150% of it.
Is deferred interest a balance transfer offer?
No. Deferred interest is a purchase-financing promotion, usually on a retail or store card, applied to a specific purchase. Balance transfer promotions move existing debt onto a card, and the mainstream US balance transfer offers are true 0% promotions rather than deferred interest. The two products are frequently confused because both are advertised with the words no interest.
What protection exists near the end of the promotion?
Under 12 CFR 1026.53(b)(1)(i), during the last two billing cycles of a deferred interest promotion, payments above the minimum must be applied to the deferred balance first. That helps at the very end, but for every earlier cycle the ordinary highest-APR-first rule applies, so it is not a substitute for tracking the balance from the start.
Sources
- CFPB, The Consumer Credit Card Market 2025, section 7.3, retrieved 2026-09-02.
- CFPB press release, retail credit card promotions (June 8, 2017), retrieved 2026-09-02.
- 12 CFR 1026.16(h), deferred interest advertising rules, retrieved 2026-09-02.
- 12 CFR 1026.53(b)(1)(i), allocation in the last two billing cycles, retrieved 2026-09-02.
- 12 CFR 1026.7(b)(14), deferred interest deadline on periodic statements, retrieved 2026-09-02.
- National Consumer Law Center, Deceptive Bargain: The Hidden Time Bomb of Deferred Interest Credit Cards (Dec 16, 2015), retrieved 2026-09-02.