Yes, you will owe deferred interest. Paying $150 a month leaves $300 unpaid when the 6-month promo ends, so the full $124 in deferred interest is charged at once, bringing your balance to $424.
The math
Inputs
| Purchase | $1,200 |
|---|---|
| APR on the card | 29.99% |
| Promotion | no interest if paid in full in 6 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 | $29.99 | $29.99 | $1,050.00 |
| 2 | $26.24 | $56.23 | $900.00 |
| 3 | $22.49 | $78.72 | $750.00 |
| 4 | $18.74 | $97.47 | $600.00 |
| 5 | $15.00 | $112.46 | $450.00 |
| 6 | $11.25 | $123.71 | $300.00 |
Result
- Balance left when the promotion ends: $300
- Deferred interest charged all at once: $124
- Balance after the interest is added: $424
- 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 $124 charge means for you
Because $300 of the $1,200 purchase is still unpaid when the 6-month promotion ends, the card issuer charges the entire deferred-interest total at once. That is the $124 figure. It is not spread over the remaining balance or billed monthly — it lands as a single charge, taking your balance from $300 to $424.
This is the trap in a deferred-interest promotion. The promotion is often described as “no interest if paid in full,” which sounds like a 0% offer. It is not. Interest builds quietly during the promotion period, and the only way to avoid it is to owe nothing on that purchase when the promotion ends. Paying $150 a month gets you close, but close is not enough.
The $124 is roughly the cost of missing the finish line by $300. You would pay it on top of the $1,200 you already agreed to pay, so the purchase effectively costs more than the sticker price. If the balance is not cleared after the interest is added, the remaining $424 continues to accrue interest at the card's APR.
Why $150 a month falls short
The promotion gives you 6 months to pay $1,200 in full. At $150 a month, your payments do not clear the purchase by the deadline, leaving $300. The math is simple: the payment is too small for the deadline, not the interest rate.
During the promotion, interest still builds up. Each month, the balance is multiplied by the APR divided by 12 — 29.99% ÷ 12 = 2.499% a month — and that amount is added to a running deferred-interest total. Because the balance stays higher for longer under a $150 payment, the running total reaches $124 by the end of the promotion. If any part of the purchase is unpaid at that point, the whole running total is charged at once.
The key driver is the unpaid balance at the deadline, not the size of the monthly payment alone. A payment that clears the purchase in time avoids the deferred interest entirely. The numbers here show that payment is $200.00. At $200.00 a month, the balance reaches $0 by the end of the promotion, so none of the deferred interest is charged.
How to change the outcome with the numbers you have
You have two levers: the monthly payment and the deadline. Raising the payment from $150 to $200.00 clears the $1,200 purchase within the 6-month promotion, so the $124 deferred interest is never charged. That is the cleanest fix, and it uses only the numbers in this scenario.
If $200.00 a month is not possible, the other option is to pay off the remaining $300 before the promotion ends. You could do that with a larger final payment or by adding extra to earlier payments. The goal is the same: get the promotional balance to $0 by the deadline. Any amount still owed when the promotion ends triggers the full $124 charge, not a prorated portion.
Keep in mind that issuers typically calculate interest on the average daily balance, so the real deferred-interest figure can differ a little from $124. Payments are also assumed to go to the promotional purchase with no other charges on the card. If you have other balances or new charges, the outcome can change. Confirm the exact payoff amount and deadline with your card issuer before the promotion ends.
Practical next steps
Start by checking the promotion end date and the current promotional balance. Then decide which path fits your budget:
- Pay $200.00 a month to clear the $1,200 purchase within the 6-month promotion and avoid the $124 deferred interest.
- Keep paying $150 a month but plan to pay the remaining $300 before the deadline, so the balance is $0 when the promotion ends.
- Confirm the payoff figure with your card issuer shortly before the deadline, since average daily balance calculations can make the exact amount differ slightly.
If you do nothing different, expect the $124 deferred interest to be added all at once, leaving a $424 balance. That balance can then accrue interest at the card's APR. These are estimates that show how the math works, not financial advice. Your card issuer's terms control, so verify the promotion rules and payoff amount with them.
Frequently asked questions
What happens if I pay $150 a month and the promotion ends?
Your payments will not clear the $1,200 purchase, leaving $300 unpaid. Because the balance is not $0 when the promotion ends, the full $124 deferred interest is charged at once, bringing the balance to $424. That remaining balance can then accrue interest at the card's APR.
Is deferred interest the same as 0% APR?
No. With a true 0% intro APR, no interest builds up during the promotion. With a deferred-interest promotion, interest still builds up each month and is charged all at once if any part of the purchase is unpaid when the promotion ends. Paying in full by the deadline avoids the charge.
How much do I need to pay each month to avoid the $124?
The payment that clears the $1,200 purchase within the 6-month promotion is $200.00 a month. At that level, the balance reaches $0 by the deadline, so none of the deferred interest is charged. Paying less leaves a balance and triggers the full deferred-interest total.
Can the deferred interest amount differ from $124?
Yes, it can differ a little. Issuers typically calculate interest on the average daily balance, and the $124 figure is an estimate based on the stated assumptions. Payments are also assumed to go to the promotional purchase with no other charges on the card. Confirm the exact amount with your card issuer.