Deferred Interest Promo Cost Calculator

How it is calculated

The tool charges simple interest on the whole original purchase for the whole promo period: retroactive interest = balance × APR ÷ 100 × months ÷ 12. The total owed if the promo is missed is balance + retroactive interest, and the payment needed to clear it in time is balance ÷ months. The APR to enter is the card's regular rate that would be applied back to the purchase date, not the 0% promotional rate. The promo length must be a whole number of months, from 1 to 120, and the APR must be above 0 and no more than 100; values outside these limits, or a balance of zero, show no result. The tool does not compound the interest, reduce the balance as you pay, or add fees, so it shows the worst case of an untouched balance; the card's own statement figure may differ.

Common mistakes

• Entering 0 as the APR because the promotion is called interest-free. The calculation needs the card's regular APR, the rate that is charged back if you miss the deadline, and a zero APR returns nothing.
• Assuming the interest applies only to what is left unpaid. With deferred interest it is charged on the original purchase for the whole promo period, which is what this tool shows.
• Entering a promo length such as 18.5 months or a day count. The promo period must be a whole number of months, so convert it first; 12.5 shows no result.
• Dividing the balance by the months and stopping there. The monthly figure is the amount needed to clear it by the end date, and the card's minimum payment is often lower, so paying only the minimum can leave a balance.
• Treating the figure as an exact bill. The tool uses simple interest and ignores compounding, partial payments and fees, so the issuer's number may differ.