If you pay $150 a month on a $2,400 purchase at 29.99% APR and don't clear it in 12 months, you'll owe $472 in deferred interest. That's added all at once, bringing your balance to $1,072.
The math
Inputs
| Purchase | $2,400 |
|---|---|
| APR on the card | 29.99% |
| Promotion | no interest if paid in full in 12 months |
| Monthly payment | $150 |
The formula in words
During the promotion interest still builds up: each month, balance × APR ÷ 12 (29.99% ÷ 12 = 2.499% a month) is added to a running deferred-interest total. If any of the purchase is unpaid when the promotion ends, the whole running total is charged at once. If the balance is $0 by then, none of it is.
Step by step
| Month | Interest this month | Deferred interest so far | Balance after payment |
|---|---|---|---|
| 1 | $59.98 | $59.98 | $2,250.00 |
| 2 | $56.23 | $116.21 | $2,100.00 |
| 3 | $52.48 | $168.69 | $1,950.00 |
| 4 | $48.73 | $217.43 | $1,800.00 |
| 5 | $44.99 | $262.41 | $1,650.00 |
| 6 | $41.24 | $303.65 | $1,500.00 |
| 7 | $37.49 | $341.14 | $1,350.00 |
| 8 | $33.74 | $374.88 | $1,200.00 |
| 9 | $29.99 | $404.87 | $1,050.00 |
| 10 | $26.24 | $431.11 | $900.00 |
| 11 | $22.49 | $453.60 | $750.00 |
| 12 | $18.74 | $472.34 | $600.00 |
Result
- Balance left when the promotion ends: $600
- Deferred interest charged all at once: $472
- Balance after the interest is added: $1,072
- Payment that clears the purchase in time: $200.00
Assumptions
- A deferred-interest promotion ("no interest if paid in full"), common on store cards. A true 0% intro APR is different: no interest builds up during the promo.
- Issuers calculate on the average daily balance, so the real figure can differ a little.
- Payments go to the promotional purchase and there are no other charges on the card.
- These are estimates that show how the math works, not financial, tax or legal advice. Lenders, card issuers and the IRS can calculate slightly differently.
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What the $472 deferred interest charge means for you
When the 12-month promotion ends, you still owe $600 on the purchase. Because the balance isn't zero, the card issuer charges the entire deferred interest total of $472 at once. That single charge pushes your balance from $600 to $1,072. In other words, the $150 monthly payments you made didn't just fail to clear the purchase — they also left you exposed to a lump-sum interest bill that's nearly as large as the remaining balance itself.
This is the trap in a deferred-interest promotion. You don't pay interest month by month, so the payments feel manageable. But interest is still building quietly in the background. The moment you miss the payoff deadline by even a small amount, the whole accumulated charge lands on your statement. That's why the balance after the interest is added, $1,072, is so much higher than the $600 you still owed on the purchase.
If you can't pay the full $1,072 right away, that new balance will start accruing interest under the card's regular terms. The issuer calculates interest on the average daily balance, so the real figure can differ a little from these estimates. Confirm the exact amount with your card issuer.
What drives the size of the deferred interest charge
Three things control how big that deferred interest bill gets: the purchase amount, the APR, and how long the balance stays unpaid during the promotion. Here, the purchase is $2,400 and the APR is 29.99%. Each month, the issuer adds 2.499% of the balance to a running deferred-interest total. That monthly rate is simply the APR divided by 12.
Because the balance starts high and only slowly comes down with $150 payments, the interest builds quickly. By the time the promotion ends, the running total reaches $472. If the balance were smaller, or the APR lower, the deferred interest would be smaller. If you paid more each month, the balance would fall faster and less interest would build.
The key point is that the deferred interest is calculated on the balance as it declines, not on the original $2,400 for the full 12 months. That's why the charge is $472 rather than a much larger number. Still, it's a real cost that hits all at once. The issuer uses the average daily balance, so your actual charge may vary slightly. Check with your card issuer for the exact figure.
How you could change the outcome with the numbers you have
The results show one clear way to avoid the deferred interest entirely: pay $200.00 a month instead of $150. At $200.00 a month, the purchase is cleared in time, the balance is $0 when the promotion ends, and none of the deferred interest is charged. That's the difference between owing $472 and owing nothing extra.
If $200.00 a month isn't possible, you could aim to pay off the remaining $600 before the promotion ends. The deferred interest is only charged if any part of the purchase is unpaid when the promotion ends. So even a smaller monthly payment works if you make a lump-sum payment to bring the balance to $0 by the deadline. The math doesn't care how you get there — it only cares whether the balance is $0 at the end.
Another option is to pay more than $150 when you can, even if you can't commit to $200.00 every month. Every extra dollar reduces the balance and the running deferred-interest total. But be careful: if you still have a balance when the promotion ends, the whole deferred interest total is charged, not just the interest on the unpaid amount. Confirm your card's specific terms with the issuer.
Practical next steps to protect yourself
First, find out exactly when your promotion ends. Mark that date and work backward. If you're paying $150 a month, you'll still owe $600 at the end, so you need a plan to cover that remaining balance before the deadline.
Second, check your statement each month to see how much of the purchase is left. The issuer may show the deferred interest total separately, or you may need to call and ask. Knowing the running total helps you decide whether to pay more.
Third, if you can't clear the balance in time, consider whether paying the $472 deferred interest plus the $600 balance is manageable. If not, you might need to adjust your budget or look for other ways to pay down the debt. These are estimates, not financial advice. Your card issuer can give you the exact payoff amount and deadline.
Finally, remember that a true 0% intro APR is different from a deferred-interest promotion. With a true 0% offer, no interest builds up during the promo. With deferred interest, the clock is always running. If you're unsure which type you have, ask your issuer.
Frequently asked questions
What happens if I pay off the balance before the 12 months end?
If the balance is $0 by the time the promotion ends, none of the deferred interest is charged. In this example, paying $200.00 a month clears the purchase in time, so you'd owe nothing extra. If you pay $150 a month, you'd still owe $600 and the full $472 would be charged.
Can I avoid the deferred interest by paying the remaining $600 at the end?
Yes. The deferred interest is only charged if any part of the purchase is unpaid when the promotion ends. If you pay the remaining $600 before the deadline, the balance is $0 and the $472 is not charged. The key is to make that payment before the promotion expires.
Why is the deferred interest $472 and not just interest on the $600 left?
Deferred interest builds on the balance throughout the promotion, not just on what's left at the end. Each month, 2.499% of the balance is added to a running total. By the end, that total is $472. If any balance remains, the whole $472 is charged at once.
Does the deferred interest charge affect my credit score?
The deferred interest itself doesn't directly affect your credit score. But if you can't pay the new $1,072 balance and it leads to late payments or high credit utilization, that can hurt your score. The charge is added to your balance, so it may increase how much credit you're using.
How can I tell if my card has deferred interest or a true 0% intro APR?
Check your card agreement or call your issuer. A deferred-interest promotion says 'no interest if paid in full' and charges all accrued interest if you don't pay in full. A true 0% intro APR charges no interest during the promo period. The terms are different, so confirm which one you have.