If you pay $300 a month on a $4,000 medical credit card charge at 26.99% APR and don't clear it within 12 months, you'll owe $634 in deferred interest. That's charged all at once when the promotion ends, leaving a $400 balance that jumps to $1,034.
The math
Inputs
| Purchase | $4,000 |
|---|---|
| APR on the card | 26.99% |
| Promotion | no interest if paid in full in 12 months |
| Monthly payment | $300 |
The formula in words
During the promotion interest still builds up: each month, balance × APR ÷ 12 (26.99% ÷ 12 = 2.249% 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 | $89.97 | $89.97 | $3,700.00 |
| 2 | $83.22 | $173.19 | $3,400.00 |
| 3 | $76.47 | $249.66 | $3,100.00 |
| 4 | $69.72 | $319.38 | $2,800.00 |
| 5 | $62.98 | $382.36 | $2,500.00 |
| 6 | $56.23 | $438.59 | $2,200.00 |
| 7 | $49.48 | $488.07 | $1,900.00 |
| 8 | $42.73 | $530.80 | $1,600.00 |
| 9 | $35.99 | $566.79 | $1,300.00 |
| 10 | $29.24 | $596.03 | $1,000.00 |
| 11 | $22.49 | $618.52 | $700.00 |
| 12 | $15.74 | $634.27 | $400.00 |
Result
- Balance left when the promotion ends: $400
- Deferred interest charged all at once: $634
- Balance after the interest is added: $1,034
- Payment that clears the purchase in time: $333.33
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 $634 deferred interest charge actually means for you
Deferred interest is not the same as a true 0% intro offer. During the promotion, interest still builds up quietly in the background. Each month, 2.249% of your balance is added to a running total. If you pay the purchase off in full before the promotion ends, that running total disappears. If you don't, the whole thing is charged at once.
In your case, paying $300 a month for 12 months leaves $400 of the original $4,000 purchase unpaid. That $400 is enough to trigger the entire deferred interest charge of $634. The card issuer adds it to your balance in one lump, so your remaining $400 suddenly becomes $1,034. That is the trap: you made steady payments toward the purchase, but you still end up owing more than a thousand dollars because of the interest that piled up behind the scenes.
The $634 is not spread out over future months. It hits your statement as a single charge the moment the promotional period ends. From there, the card's normal interest rules apply to the new $1,034 balance, which can make the debt grow even faster if you only make minimum payments. This is why deferred interest promotions are risky for anyone who cannot guarantee paying the full purchase off in time.
What drives the size of the deferred interest charge
Three things control how much deferred interest you owe: the original purchase amount, the APR, and how much of the purchase is still unpaid when the promotion ends. Your purchase was $4,000, your APR is 26.99%, and your monthly payment of $300 left $400 unpaid after 12 months. Because any unpaid balance triggers the full deferred interest, the $400 leftover is what causes the entire $634 charge.
The monthly interest build-up is based on your balance during the promotion. The calculation uses 26.99% divided by 12, which is 2.249% per month. Each month, that percentage is applied to the balance, and the result is added to the deferred interest total. As you pay down the balance, the monthly interest added gets smaller, but the total still grows every month until the promotion ends.
If you had paid $333.33 a month instead of $300, the balance would reach $0 by the end of the promotion. In that case, none of the deferred interest would be charged. The difference between $300 and $333.33 a month is small, but it is the difference between owing $634 and owing nothing extra. That is how narrow the margin can be.
How to change the outcome using the numbers you have
You cannot change the APR or the original purchase amount now, but you can change how much is left unpaid when the promotion ends. The payment that clears the purchase in time is $333.33 a month. If you can raise your monthly payment from $300 to $333.33, you would pay off the $4,000 within the 12 months and avoid the $634 deferred interest charge entirely.
If you cannot manage $333.33 every month, you could try to make up the difference with a lump sum before the promotion ends. For example, if you pay $300 a month for 11 months, you would still owe part of the purchase. Paying that remaining amount before the 12 months are up would also clear the purchase and avoid the deferred interest. The key is that the balance must be $0 by the time the promotion ends.
Another option is to pay more than $300 in the months when you can, which reduces the balance faster and lowers the monthly interest that builds up. Even small extra payments can help, but you still need to reach a $0 balance by the deadline. If you are not sure you can do that, contact your card issuer to confirm exactly how they calculate the deferred interest and when the promotion ends. The numbers here are estimates, and your issuer may calculate on the average daily balance, so your actual charge could differ a little.
Practical next steps to avoid or manage the $634 charge
First, find out the exact end date of your deferred interest promotion. Mark it on your calendar and set a reminder a month before. You need time to adjust your payments if you are falling short. Second, check your current balance and compare it to the $400 leftover in this example. If you are on track to have any unpaid balance when the promotion ends, you will trigger the full deferred interest charge.
Third, decide how you will close the gap. If you can pay $333.33 a month, switch to that amount now. If not, plan a lump sum payment before the deadline. You could also call your card issuer and ask them to confirm your payoff amount and the exact date you need to pay it. They can tell you how they apply payments and whether any fees or other charges are on the card.
Fourth, if you cannot pay off the purchase in time, consider whether you can move the balance to a lower-rate option. But be careful: moving a balance may have its own costs, and the deferred interest charge may still apply if the promotion ends before you transfer. Ask your issuer and any new lender about the details before you act. Finally, remember that this is an estimate, not financial advice. Confirm all numbers with your card issuer or a financial professional before making decisions.
Frequently asked questions
What happens if I pay off the $4,000 before the 12 months end?
If the balance reaches $0 before the promotion ends, none of the deferred interest is charged. In this example, paying $333.33 a month would clear the purchase in time, so you would owe $0 in deferred interest instead of $634.
Can I avoid the $634 charge by paying a little extra each month?
Only if the extra payments bring the balance to $0 by the end of the promotion. Paying $300 a month leaves $400 unpaid, which triggers the full $634. You need to pay at least $333.33 a month or make a lump sum to clear the balance.
Is deferred interest the same as a 0% intro APR?
No. With a true 0% intro APR, no interest builds up during the promotion. With deferred interest, interest still builds up each month at 2.249% of your balance, and if any part of the purchase is unpaid when the promotion ends, the whole amount is charged at once.
Why is the deferred interest $634 when I only have $400 left?
The $634 is the total interest that built up over the 12 months on your declining balance. It is not based only on the $400 leftover. Because you did not pay the full $4,000, the entire deferred interest total is charged, turning your $400 balance into $1,034.
Will my card issuer calculate the same $634?
Possibly not exactly. Issuers often use the average daily balance method, so your actual deferred interest could be slightly different. The $634 is an estimate based on a simple monthly calculation. Check with your card issuer for the exact figure.