What credit card interest actually is
Credit card interest is the fee your issuer charges for letting you carry a balance past your due date. It's expressed as an APR — annual percentage rate — but it's applied monthly. To get the monthly rate, divide the APR by 12: a card with a 24% APR charges roughly 2% interest per month on your outstanding balance.
This calculator shows what that interest looks like in real dollars, both for a single month and compounded over a longer period if no payments are made — which is a useful way to understand exactly how expensive it is to simply "let a balance sit."
How the calculation works
We take your balance and multiply it by your monthly rate (APR ÷ 12) to get your first month's interest charge. If you extend the period and no payments are made, that interest gets added to the balance, and the next month's interest is calculated on the new, larger total — this is compounding, and it's why unpaid credit card debt grows faster the longer it's left alone.
A worked example
A $5,000 balance at 24.99% APR generates about $104 in interest in the very first month alone. If left completely unpaid for a full year, compounding monthly, the balance would balloon to roughly $6,415 — meaning over $1,415 in interest charges on top of the original $5,000, without a single new purchase.
This is why even a small regular payment makes such a big difference: it's not just reducing your balance, it's preventing that compounding cycle from taking hold.
What to do with this number
- If the monthly interest amount is close to or larger than your typical payment, you're likely stuck in — or heading toward — the minimum payment trap.
- Use the Credit Card Payoff Calculator to see how a specific monthly payment eliminates this interest over time instead of letting it compound.
- Compare your current APR against a lower one with the APR Comparison Calculator to see how much a rate reduction would save.