The arithmetic in one line
A balance transfer is worth it when the transfer fee is less than the interest you would otherwise pay during the promotional period. At 25% APR, a $9,000 balance accrues roughly $1,900 in interest over 18 months. A 3% fee is $270. The comparison is rarely close.
Set the payment from the deadline, not the minimum
Divide the transferred balance by the number of promotional months to get the payment that clears it exactly in time. That is the number to automate. The card's minimum payment is deliberately far lower and is designed to leave a balance when the promotional rate expires.
Rules that catch people out
- New purchases. On some cards these do not share the 0% rate, and payments may be applied to the promotional balance first — leaving purchases accruing interest.
- Late payments. A single late payment can void the promotional rate entirely on many cards.
- Transfer windows. The offer often requires the transfer within 60 to 120 days of opening the account.
- Limits. Your approved credit line may be smaller than the balance you wanted to move.
Deferred interest is not the same thing
Store cards frequently advertise "no interest if paid in full within 12 months". If any balance remains at the deadline, interest is charged retroactively from day one. A true 0% balance transfer card does not do this — but read the terms rather than assuming.
The behaviour that decides the outcome
A transfer only helps if the original card is not used again. Freeing up a $9,000 limit and running it back up converts one debt into two. Move the balance, then put the old card somewhere inconvenient and set an automatic payment on the new one.