What happens when a 0% promo rate ends

A promotional rate is not a discount. It is a deadline. Whether missing that deadline costs you a little or a great deal comes down to one word almost nobody is told: waived, or deferred.

Waived versus deferred

These sound like the same thing and they are not.

Waived interest (typical of a 0% purchase or balance-transfer APR on a normal credit card): interest simply does not accrue during the promotional window. When it ends, the standard rate applies to whatever is left. What you did not pay is gone for good.
Deferred interest (common on store cards and "12 months no interest" financing): interest has been accruing quietly the whole time. If any balance remains on the end date — even a dollar — the entire accrued amount is added to your balance at once.

On a $3,000 deferred-interest purchase at 27%, a $20 leftover balance can trigger several hundred dollars of back-interest in one statement. This is legal, it is disclosed in the agreement, and it catches people every single year.

The question to ask, in these words: "Is the interest during the promotional period waived, or deferred?" Ask it on a recorded call, write down the answer and the date.

The other trap: payment allocation

If you have a promotional balance and new purchases on the same card, your payments are generally applied to the promotional balance first (above the minimum, issuers must apply extra to the highest-rate balance, but the mechanics still work against you here). The practical result is that new spending sits there accruing at the standard rate while you pay down the 0% part.

The fix is unglamorous: stop using a card that carries a promotional balance. One card, one job.

The 60-day checklist

Find the exact end date. It is on the statement, not the marketing. Put it in your calendar with a 60-day warning.
Work out the clearing payment. Remaining balance divided by the months left. That is the real minimum, regardless of what the statement calls the minimum.
Ask whether it is deferred. If it is, treat the date as immovable and the clearing payment as non-negotiable.
If you cannot clear it, ask for a fixed-rate payment plan before the promotion ends, and ask what the post-promotional rate will be. Comparing that rate against your other options is a decision to make in advance, not in the month the interest lands.
Do not open a second promotional offer to move it without adding up the transfer fee (typically 3–5%) plus the new rate afterwards. Sometimes it is worth it. Often it is a way of paying to delay.

What not to do

Do not assume the minimum payment clears the balance in time. On deferred-interest plans it frequently does not, and that is not an accident.
Do not put new purchases on the card.
Do not wait for a reminder. Nobody has an incentive to send you one.

Our assessment flags every promotional rate you have entered with the date it ends, and works out the payment that clears it in time — because that deadline is the one part of your plan set by somebody else.

This is financial education, not legal, tax or investment advice, and it is not a recommendation for your specific situation. Rescue My Finances is not a credit repair organization, debt settlement company or credit counseling agency. See our disclosures.