What is compound interest?
When you borrow money, lenders charge you for using their funds. With simple interest, that interest is calculated on the original amount you borrowed. With compound interest, which is what most credit cards charge, interest is added to your balance, and the next time interest is calculated it can be based on that new, larger amount. The longer you carry the balance, the more it can grow.
Why compound interest matters for a large purchase
Compound interest matters because the price you see at checkout isn’t necessarily the amount you’ll ultimately pay if you carry the purchase as an interest-bearing balance. The longer interest is added to an unpaid balance, the more the purchase can cost. That’s why it’s useful to compare the total cost of different ways to pay – not just the size of the monthly payment.
How compound interest is calculated
Here’s a simplified example using annual compounding to show the effect.
Imagine you make a $1,000 purchase with 20% annual interest. For this simplified example, we’ll assume the interest compounds annually.
| Year | Starting Balance | 20% Interest Added | Ending Balance |
|---|---|---|---|
| Year 1 | $1,000.00 | $200.00 | $1,200.00 |
| Year 2 | $1,200.00 | $240.00 | $1,440.00 |
| Year 3 | $1,440.00 | $288.00 | $1,728.00 |
| Year 4 | $1,728.00 | $345.60 | $2,073.60 |
Notice how the interest gets larger every year? In Year 1, the interest was $200. By Year 4, the interest alone was $345.60 because it is growing by 20% on the new balance, not just the original $1,000.
Of course, this example assumes no payments are made along the way — real credit cards require a minimum payment each month. That minimum is typically a flat percentage of your balance, a fixed dollar amount, or interest plus fees, whichever is greater, and by law it has to at least cover the interest charged that month. If you’re paying close to the minimum, you may be covering little more than the interest itself, leaving the principal to clear at a crawl.
Ways to avoid being charged interest on your credit card
Depending on your card and its terms, paying your statement balance in full by the due date can help you avoid paying interest on purchases.
- Pay in full every month: Depending on your card and its terms, paying your statement balance in full by the due date can help you avoid paying interest on purchases.
- Avoid cash advances: Taking cash out on a credit card starts charging interest from the day of the withdrawal, with no grace period and usually at a higher rate than purchases.
Credit card installments and compound interest
If you don’t want to pay for a larger purchase all at once, one option may be to divide it into scheduled installments. With credit card installments, you can use the available credit on a card you already have and spread the purchase across a defined payment schedule. This gives you a clearer view of the payments and when the plan will end, rather than simply leaving the purchase as an open-ended balance.