A 0% balance transfer looks like a simple trade. You pay a fee, usually 3% to 5% of the amount you move, and in exchange the new card charges no interest on that debt for a year or more. Most explanations of how balance transfers work stop there, at the fee and the promo length. But the fee is the part of the deal you can see. The cost that catches people sits in two quieter rules: one that controls where your payments go, and one that quietly switches off the interest-free window on everything new you buy.
With the average rate on credit card accounts that pay interest at 22.36% in August 2026, according to the Federal Reserve’s G.19 data as charted by FRED, and with January statements about to show holiday spending, a lot of people will be looking at 0% offers in the next few weeks. Before you take one, it helps to know which parts of the deal the issuer is counting on.
Issuers can lose money on the promo, which tells you where the profit comes from
Start with a question most articles skip: why would a bank lend you money at zero percent?
Economists Lukasz Drozd of the Philadelphia Fed and Michal Kowalik of the Boston Fed studied exactly this, using supervisory data from 2018 and 2019 that covered about 70% of US credit card accounts. Their findings, summarized by the Philadelphia Fed in 2023, are a little surprising. About a quarter of all credit card debt was sitting on an introductory promotional rate. When a promotion ended, the APR jumped by 16 percentage points on average. And under a typical 12-month 0% deal with a 3% transfer fee, most lenders lost money during the promotional period itself. The fee, the authors wrote, “appears grossly insufficient to cover the default losses.”
So the fee isn’t the business model. The issuer is betting on what happens around the promo: balances that outlive the 0% window, and new spending on the card at the regular rate. The researchers’ explanation is that people tend to underestimate their future card spending. Hold onto that idea, because both of the rules below work in the issuer’s favor when that underestimate comes true.
The grace period disappears the moment you carry a transferred balance
Every credit card with a grace period lets you avoid interest on purchases, but only if you pay the full statement balance by the due date. Most people think of that as “pay off what I bought.” The card thinks of it as “pay off everything,” and everything includes the transferred balance.
Citi spells this out in the terms for its Double Cash card, which offers 0% on balance transfers for 18 months but not on purchases. Citi’s offer page says that if you transfer a balance, interest will be charged on your purchases unless you pay your entire balance, including balance transfers, by the due date each month. Since the whole point of a transfer is that you’re paying it down over many months, you will not be paying the entire balance. That means every new purchase starts accruing interest from the day it posts.
This is the part people miss. You can pay your new spending in full every single month and still get charged interest on it, because the grace period is tied to the total balance, not to the purchases.
Your payments are sorted by law, and the minimum is the issuer’s choice
The second rule comes from federal law. Under Regulation Z, section 1026.53, any amount you pay above the required minimum must go first to the balance with the highest APR. That’s a consumer protection added by the CARD Act, and it mostly helps you here: extra money goes to your interest-bearing purchases before the 0% balance.
There’s a wrinkle, though. The official commentary to the rule says it doesn’t limit how the issuer allocates the minimum payment itself. An issuer is free to apply your minimum to the lowest-rate balance first, which on a transfer card is the 0% debt. The rule also has a special exception for deferred-interest plans (the “no interest if paid in full” kind you see at furniture stores), where extra payments shift toward the promo balance in the final two billing cycles before it expires. A true 0% balance transfer isn’t deferred interest, so if you don’t pay it off, interest starts on the leftover balance going forward rather than being charged back to the start.
Put the two rules together and you can see the design. New purchases accrue interest immediately because the grace period is gone. Your payments do eventually reach those purchases, but the interest has been accruing every day since each one posted, and the minimum may have gone to the 0% balance anyway.
A worked example: how balance transfers work on a $6,000 balance
Say you’re carrying $6,000 on a card at the 22.36% average rate and you can afford $350 a month. Left alone, that balance takes 21 months to clear and costs about $1,296 in interest.
Now move it to the Citi Double Cash within the first four months, when the transfer fee is 3%. The fee is $180, so your new balance is $6,180. At $350 a month, you’re done in 18 payments, just inside the 0% window. Your total cost is the $180 fee, which saves you roughly $1,116. That’s the version of the math every comparison site shows.
Now add one ordinary habit. Suppose you also keep using that card for about $800 a month of groceries and gas, and you pay that $800 in full every month on top of your $350. Because you’re carrying the transferred balance, those purchases get no grace period. If each dollar sits on the card for about 40 days on average (half a billing cycle until the statement, plus roughly 25 days until the due date), then at Citi’s lowest regular rate of 18.49% you pay about $800 × 18.49% × 40/365, or around $16 a month. Over 18 months that’s about $292. At the top of the range, 28.74%, it’s about $453. Your $1,116 savings shrinks to somewhere between roughly $660 and $820, and you never once missed a payment.
The fix is simple once you see the mechanism: put everyday spending on a different card, or a debit card, while the transfer is being paid down. Some cards, like the Wells Fargo Reflect, offer 0% on both purchases and transfers for 21 months (with a 5% transfer fee, per Experian’s card listing), which removes the purchase problem during the promo. That’s a different trade, and the higher fee is the price of it.
What actually decides whether a balance transfer pays off
How balance transfers work comes down to timing and separation. The fee is a known, one-time cost. The expensive parts are the balance that’s still there when the promo ends (when your rate can jump by double digits, as the Fed researchers found) and the purchases you keep making on a card that has quietly stopped giving you a grace period. If you can pay the transferred balance off inside the window and keep new spending somewhere else, the 0% offer is a real deal. If you can’t do both, the issuer is likely to make back what it gave up.
For more on what happens to the rate after the promo, see our explainer on how credit card APRs are set from the prime rate plus a margin, and on how minimum payments are calculated.
