The offer has three numbers, and two get ignored
Every balance transfer offer contains a promotional rate, a promotional length, and a transfer fee. The advertising leads with the first, prints the second in smaller type, and mentions the third in the terms.
- The promotional rate is commonly 0%, and it is genuinely 0%. This part is not a trick.
- The promotional length is a fixed number of billing cycles from account opening or from the transfer, and it varies widely between offers. It is the number that decides everything, so read it off the offer itself rather than from an advertisement.
- The transfer fee is a percentage of the amount moved, charged once, up front, and added to the balance. Low single digits is the usual territory. A small number of offers waive it, usually in exchange for a shorter promotional window.
The fee is the true price of the offer, and it is paid whether or not the strategy works. That makes the arithmetic simple: a transfer is worth doing when the interest it avoids over the promotional period exceeds the fee. On a high-rate balance you intend to attack hard, that is an easy yes. On a balance you were going to clear in four months anyway, a fee for a promotion you barely use is money handed over for nothing.
What the cliff actually is
When the promotion ends, any remaining balance begins accruing at the card's ordinary purchase or transfer rate — the go-to rate, disclosed in the offer. Nothing is charged retrospectively. The interest that did not accrue during the promotion does not come back.
The damage is not retrospective, then. It is that you have arrived at the end of a fixed window still holding a balance, now on a card whose ordinary rate may be no better than the one you left, and the fee is already spent.
$9,000 transferred with a 3% fee: $270, added on. You now owe $9,270 at 0% for 18 billing cycles.
To clear it inside the window: $9,270 ÷ 18 = $515 a month, every month, without interruption. That is the number the offer never puts in front of you.
Pay the minimum instead — on a card calculating it as 2% of the balance, with no interest to add — and the balance falls to roughly $6,400 by the final cycle. If the go-to rate were 24%, that balance starts costing about $129 in its first month.
The transfer was not wasted: eighteen months of interest was genuinely avoided. But the household that paid the minimum is now two-thirds of the way into an unchanged problem, $270 lighter, and out of road.
Figures are illustrative and the minimum-payment formula, the promotional length and the go-to rate all come from your own offer. The route comparison runs this against the other four routes with your numbers, and shows what is left standing at the cliff rather than assuming you make it.
Deferred interest is a different thing, and it is worse
Store cards and retail financing frequently advertise "no interest if paid in full within twelve months". That is deferred interest, not a promotional rate, and the distinction is the most expensive one in consumer credit.
Under a true 0% promotion, interest simply does not accrue during the window, and only the remaining balance accrues afterwards. Under deferred interest, interest accrues the whole time and is merely held back — and if any balance at all remains at the deadline, the entire accumulated amount is charged, calculated from the original purchase date. Being $40 short in the final month can trigger a year's interest on the full amount.
The way to tell them apart is the wording. "0% APR for 18 months" is a promotional rate. "No interest if paid in full by" is deferred interest. If you have one of the latter, put the payoff date in your calendar with a fortnight to spare.
Four things that end a promotion early or blunt it
- Serious delinquency. Card agreements typically allow the promotional rate to be withdrawn once a payment is more than sixty days late, at which point the whole plan collapses. Set the payment up automatically.
- New purchases on the same card. Federal rules require payments above the minimum to be applied to the highest-rate balance first, which sounds protective and mostly is — but it means your extra payments go to the purchases at the ordinary rate while the promotional balance sits still. Whether new purchases even get an interest-free grace period while a promotional balance is outstanding depends on the agreement. The simplest answer is to use the transfer card for the transfer and nothing else.
- A transfer window. The promotional fee and rate often apply only to transfers made within a set period after opening the account. Move the balance promptly.
- An approved amount smaller than you asked for. The issuer sets the limit, and a partial transfer leaves part of the balance where it was, at the old rate, which changes the whole comparison.
Practical points that are easy to get wrong
You usually cannot transfer between cards from the same issuer. Check before applying.
Keep paying the old card until the transfer lands. Processing can take days or longer, and a missed payment in the gap undoes the exercise.
Leave the old card open with a zero balance. Closing it removes its limit from your available credit and raises your utilisation. Leaving it open and unused is better for your file — and worse for your willpower, which is a real consideration and only you know the answer.
Expect a short-term score effect. A new account lowers your average account age and adds an enquiry. The improvement in utilisation usually outweighs it, but if a mortgage application is imminent, do nothing to your credit file without asking your loan officer first — see what underwriting looks at besides the ratio.
The one test
Before accepting an offer, work out the balance including the fee, divide by the number of promotional cycles, and ask whether you can pay that amount every month without fail. If yes, a transfer is one of the cheapest tools available to a household with high-rate card debt. If no, you are paying a fee for a delay, and the honest comparison is against a consolidation loan, an issuer hardship programme, or simply attacking the balance where it sits.
General description of United States credit card promotional balance transfer terms and the payment allocation and penalty-rate rules introduced by the Credit CARD Act of 2009. Fees, promotional lengths, go-to rates, minimum payment formulas and grace period treatment are set by your individual card agreement and vary widely. Worked figures are illustrative only. Not financial, credit or debt advice.