Credit card interest = your average daily balance × your daily periodic rate × days in the billing cycle. A $3,000 balance at 24% APR costs you about $60 a month in interest — and that number grows every month you only pay the minimum.
The 30-second version
- Your card company divides your APR by 365 to get a daily rate.
- Each day, it multiplies that rate by your balance that day, then adds up all 30-ish days.
- Pay the full statement balance by the due date and you pay zero interest on purchases. That's the grace period doing its job.
- Pay only the minimum and the math turns brutal — most of your payment goes to interest, barely touching what you owe.
The single sentence that matters: interest is charged on your average daily balance, not your starting balance. Big purchase on day 25 of the cycle? You only pay interest on it for a few days.
Do the math with me
Say your APR is 24% and you carry a $3,000 balance through a 30-day billing cycle.
Step 1: Daily periodic rate = 24% ÷ 365 = 0.0658% per day.
Step 2: $3,000 × 0.000658 = $1.97 per day.
Step 3: $1.97 × 30 days = $59.18 in interest for the month.
Now here's where it stings: next month you're paying interest on $3,059.18. The month after, on that plus more interest. This is compounding working against you — the same force that grows savings accounts, now growing your debt.
| Balance carried | Monthly interest at 24% APR |
|---|---|
| $1,000 | $19.73 |
| $3,000 | $59.18 |
| $5,000 | $98.63 |
| $10,000 | $197.26 |
Tip: Made a big purchase mid-cycle? Pay it off before the statement closes and it barely accrues interest at all — interest runs on daily balances, so days matter.
Interest at different APRs
Your rate changes the bill more than most people expect. Here's one 30-day billing cycle at common card APRs, using the same daily method your issuer uses:
| Balance | 19.99% APR | 24.99% APR | 26.99% APR | 29.99% APR |
|---|---|---|---|---|
| $1,000 | $16.43 | $20.54 | $22.18 | $24.65 |
| $3,000 | $49.29 | $61.62 | $66.55 | $73.95 |
| $5,000 | $82.15 | $102.70 | $110.92 | $123.25 |
Moving a $5,000 balance from 29.99% to 19.99% saves about $41 a month before you pay down a single dollar of principal. That's why a balance transfer or a lower-rate personal loan can be worth a look.
The monthly shortcut (and why it's slightly off)
Want a quick monthly number without counting days? Divide your APR by 12 and multiply by the balance:
Monthly interest ≈ balance × (APR ÷ 12)
For $3,000 at 24%, that's $3,000 × 2% = $60. Close to the $59.18 above, but not exact, because your card doesn't charge by the month. It charges by the day, so a 31-day cycle costs a little more than a 28-day one. Use the shortcut for a ballpark; use the daily method when you want to match your statement.
The same logic works backward. If you know your APR and want the annual cost of a balance you never pay down, it's roughly balance × APR, plus a bit more from compounding each month.
Average daily balance, step by step
Real months aren't one flat balance. You buy things, you make a payment, the number moves. Here's how the issuer handles that, using a 30-day cycle at 24% APR:
Days 1–10: balance $2,000.
Day 11: you buy a $600 laptop → balance $2,600 for days 11–20.
Day 21: you pay $500 → balance $2,100 for days 21–30.
Step 1, add up each day's balance: (10 × $2,000) + (10 × $2,600) + (10 × $2,100) = $67,000.
Step 2, divide by days in the cycle: $67,000 ÷ 30 = $2,233.33 average daily balance.
Step 3, apply the daily rate: $2,233.33 × 0.000658 × 30 = $44.05 interest.
Notice the timing effect: paying that $500 ten days earlier would have shaved the average down further. On a card you carry a balance on, paying as soon as you get paid, rather than on the due date, lowers your interest every month.
How to find your interest rate
Look on your monthly statement for a section usually titled "Interest Charge Calculation." It lists separate rows for purchases, balance transfers and cash advances, each with its own APR, the balance it applied to, and the interest charged. Many issuers also show the daily periodic rate there, so you don't have to divide by 365 yourself.
Can't find a statement? Your card's app or online account usually shows the purchase APR under account details, and the full rate table is in your cardholder agreement. Most US card rates are variable, which means they move when the prime rate moves, so the APR you signed up with may not be the one you pay today.
Cash advances cost more, fast
Withdrawing cash on a credit card is the most expensive way to use it. Three things stack up at once: a higher cash-advance APR (often near 30%), no grace period so interest starts on day one, and an upfront fee, commonly around 5% of the amount.
Take $500 at 29.99% for one 30-day cycle and you pay about $12.32 in interest, plus a $25 fee at 5%. That's roughly $37 to borrow $500 for a month. If you need short-term cash, almost anything is cheaper.
The grace period (use it or lose it)
Most cards give you roughly 21–25 days after the statement closes to pay in full with zero interest on purchases. But the grace period has a catch most people learn the expensive way: it only applies if you paid your previous balance in full.
Carry even $1 into the new cycle and the grace period vanishes — new purchases start accruing interest the day you make them. One leftover dollar can cost you a full month of interest on everything you buy next. If you've been carrying a balance, the only way back to free grace-period living is paying the whole thing to zero.
Also note: cash advances and balance transfers usually have no grace period at all. Interest starts ticking the second the money moves.
Why minimum payments are a trap
A $5,000 balance at 24% APR with a $125 minimum payment takes roughly 7 years to clear — and costs about $5,160 in interest alone. You'd pay more in interest than you borrowed.
Bump that payment to $200 and you're done in about 3 years with roughly $2,000 in interest. Same debt, same rate — the only thing that changed is how aggressively you attacked the principal. Minimum payments are designed to keep the account profitable for the lender, not to get you out of debt.
See your real payoff date
Plug in your balance, APR, and monthly payment to see the true cost.
Open the interest calculator →Quick answers
Is credit card interest charged daily or monthly?
Calculated daily, billed monthly. Your issuer applies the daily periodic rate (APR ÷ 365) to your balance each day, then totals it up on your statement.
Does paying the minimum stop interest?
No — it only keeps the account in good standing. Interest keeps accruing on the remaining balance every single day.
Why is my interest different from what I estimated?
Because it's based on your average daily balance, not the balance on any one day. Payments, purchases, and refunds during the cycle all move the number.
Do all cards use APR ÷ 365?
Nearly all US cards use 365 days. A few use 360 — check your cardholder agreement if you want the exact figure, but the difference is tiny.
What is the formula for credit card interest?
Interest = average daily balance × (APR ÷ 365) × days in the billing cycle. For a quick estimate, balance × APR ÷ 12 gets you within a dollar or two for most balances.
Do I pay interest on purchases if I pay my bill in full?
No. If you pay the full statement balance by the due date every month, the grace period means purchases cost you nothing in interest. Interest only applies to the part you carry over, plus any cash advances.
General information only, not financial advice.