The Basic Formula: Balance × (APR ÷ 12) = Monthly Interest
Your credit card company divides your annual percentage rate (APR) by 12 to get a monthly rate, then multiplies that by your current balance. That number is what you owe in interest that month.
Here's a concrete example: if your balance is $2,000 and your APR is 18%, the math looks like this: $2,000 × (0.18 ÷ 12) = $2,000 × 0.015 = $30. You owe $30 in interest that month, on top of any principal you pay down.
The catch is that your balance changes every day. Most cards use what's called the "average daily balance" method, which means they add up what you owed each day of the month, divide by the number of days, and calculate interest on that average. A few cards use the "previous balance" method (interest on last month's ending balance) or the "adjusted balance" method (balance minus payments made during the month). Your card's method is in the terms you received when you opened the account, or you can find it on your issuer's website.
Key Takeaways
- Monthly interest equals your balance multiplied by your APR divided by 12, so a $2,000 balance at 18% APR costs $30 in interest that month.
- Most cards calculate interest on your average daily balance across the entire month, not just your balance on the due date.
- Interest accrues every single day you carry a balance, even if you make a payment partway through the month.
- Paying down your balance faster reduces the number of days interest accrues, which is why paying more than the minimum saves money over time.
Why Your Balance Changes Every Day
Interest doesn't sit still. Every day you carry a balance, a small amount of interest is added to what you owe. If you make a purchase on day 5 of your billing cycle, that purchase starts accruing interest when ready. If you make a payment on day 15, the interest stops accruing on the amount you paid, but continues on what remains.
This is why the "average daily balance" method matters. Say your balance was $2,000 for the first 15 days of the month, you paid $500, and the balance was $1,500 for the remaining 15 days. Your average daily balance is ($2,000 × 15 + $1,500 × 15) ÷ 30 = $1,750. Interest is calculated on $1,750, not on $2,000 or $1,500 alone.
If you want to know your exact monthly interest charge before your statement arrives, you need to know your card's method. Call the customer service number on the back of your card and ask: "Do you use average daily balance, previous balance, or adjusted balance?" They will tell you in seconds.
How Interest Compounds When You Only Pay the Minimum
The real damage happens when you pay only the minimum. Most minimum payments cover interest and a tiny slice of principal. If you owe $2,000 at 18% APR and pay only the minimum (often around 1% to 3% of the balance), you might pay $30 to $60. If $30 is interest, you've paid $0 to $30 toward the actual debt.
Next month, your balance is still nearly $2,000. Interest accrues again. And again. At this rate, a $2,000 balance at 18% APR takes roughly 10 years to pay off if you pay only the minimum, and you'll pay more than $2,000 in interest alone.
The math changes when you pay more than the minimum. If you pay $100 instead of the minimum, $30 goes to interest and $70 goes to principal. Next month, your balance is $1,930, so interest is slightly less. The gap between what you owe and what you're paying widens, and you escape the debt much faster.
Using an Online Calculator vs. Doing It Yourself
You can calculate monthly interest by hand using the formula above, but most people find an online credit card interest calculator faster. Search "credit card interest calculator" and you'll find dozens. Plug in your balance, APR, and monthly payment, and the tool shows you how much interest you'll pay and how long it takes to pay off the card.
The advantage of doing it yourself is that you understand what's happening. The advantage of a calculator is speed and the ability to test different payment amounts. If you want to know "what if I pay $150 instead of $100?", a calculator shows you the difference in months and dollars when ready.
Either way, the number you get is an estimate. Your actual interest will vary slightly depending on the exact day your payment posts and how your card company counts days. But the estimate is close enough to guide your decision about how much to pay.
Why Different Cards Charge Different Monthly Interest on the Same Balance
Two cards with the same APR can charge different monthly interest on the same balance because of when interest starts accruing. Most cards have a grace period — usually 21 to 25 days — during which new purchases don't accrue interest if you pay your full balance by the due date. But if you carry a balance from the previous month, interest on that balance starts accruing when ready, with no grace period.
Some cards also charge interest differently on purchases, balance transfers, and cash advances. A card might charge 18% APR on purchases but 25% on cash advances. If you take a cash advance, the monthly interest on that portion is higher even though it's the same card.
The card's terms document lists all of this. If you're comparing cards or trying to understand why one card's interest charge seems higher than another's, the APR and the grace period are the two numbers that matter most.
What Happens If You Miss a Payment
Missing a payment doesn't just mean you owe interest on a larger balance. Most cards charge a late fee (often $25 to $40 for the first late payment) and may raise your APR. Some cards have a "penalty APR" that kicks in after one or two missed payments, sometimes reaching 29% or higher.
If your APR jumps from 18% to 25%, your monthly interest on a $2,000 balance goes from $30 to $41.67. That's $11.67 more per month, or $140 per year, just because of the rate increase. The late fee is on top of that.
If you miss a payment, contact your card company as soon as you realize it. Many will waive the late fee if you call before the payment is 30 days late, and some will lower the penalty APR if you make the next few payments on time. It's worth asking.
Frequently Asked Questions
Does interest accrue on my credit card every day?
Yes. Interest accrues daily on any balance you carry. Even if you make a payment partway through the month, interest continues to accrue on the remaining balance until you pay it off completely. This is why paying down your balance faster saves money — there are fewer days for interest to build up.
What's the difference between APR and monthly interest rate?
APR is the annual rate. Monthly interest rate is the APR divided by 12. If your APR is 18%, your monthly rate is 1.5%. You multiply your balance by the monthly rate (0.015) to find that month's interest charge.
Can I negotiate my APR to lower my monthly interest?
You can ask your card company to lower your APR, especially if you've had the card for years and made on-time payments. Call the customer service number on the back of your card and ask to speak with someone about your rate. They may offer a lower rate, but there's no may provide. Paying down your balance is the fastest way to reduce the interest you owe each month.
Why is my interest charge higher than I calculated?
The most common reason is that you calculated interest on your ending balance, but the card company calculated it on your average daily balance. If you made purchases throughout the month, your average daily balance is higher than your ending balance, so interest is higher. Check your statement to see which method your card uses.
If I pay my full balance before the due date, do I owe any interest?
Not on purchases, as long as you pay the full balance and your card has a grace period (most do). Interest on purchases only accrues if you carry a balance past the due date. However, if you took a cash advance or did a balance transfer, those usually start accruing interest when ready, even if you pay other purchases in full.