How Is Credit Card Interest Calculated?
Your card advertises an annual rate (APR), but the interest charge that lands on your statement is built one day at a time. Once you see the daily math, three things click into place: why carrying a balance is so expensive, why paying in full erases interest entirely, and why the timing of a payment matters. Here is exactly how the number is calculated, with worked examples you can reproduce.
Use the calculator
Credit Card Interest Calculator
Step-by-step
- 1
Start with the APR and find the daily periodic rate
Take your purchase APR and divide by 365 to get the daily periodic rate. A 22% APR becomes 22 ÷ 365 = 0.0603% per day. A 24% APR becomes 0.0658% per day. This tiny daily rate is what your balance is multiplied by every single day it stays unpaid. (Some issuers divide by 360; the difference is small.)
- 2
Calculate your average daily balance
The card records your balance at the end of each day in the billing cycle, adds new purchases, subtracts payments, then averages those daily balances across the whole cycle. If you held $5,000 for the first 15 days and $5,200 for the last 15 days of a 30-day cycle, your average daily balance is (5,000 × 15 + 5,200 × 15) ÷ 30 = $5,100.
- 3
Multiply: average daily balance × daily rate × days in cycle
This is the core formula. On a $5,000 average daily balance at 22% APR over a 30-day cycle: $5,000 × 0.000603 × 30 = about $90.45 of interest for the month. Bump the APR to 26% and the same balance costs about $106.85. The interest scales directly with both the balance and the rate.
- 4
Understand the grace period that can make interest zero
If you paid last month's statement balance in full by the due date, new purchases get a grace period — typically 21–25 days during which no interest accrues. Pay this month's statement in full too, and you owe $0 interest, no matter how much you charged. The grace period is the single most valuable feature of a credit card, and it only exists while you carry no balance forward.
- 5
See what happens when you carry a balance
Carry any balance past the due date and the grace period disappears. Now every new purchase starts accruing interest from the transaction date, and last month's leftover balance keeps compounding daily. This is why a balance feels "sticky" — interest is charged on interest, day after day, until you clear the full balance for one complete cycle.
- 6
Account for the higher APRs hiding on your card
Most cards carry at least three rates: a purchase APR (the headline number), a cash advance APR usually 5–8 points higher with no grace period, and a balance transfer APR (0% during a promo, then reverting). Cash advances and the post-promo balance-transfer rate are calculated the same daily way — just at a higher number, which is why they are so costly.
💡 Tips
- Pay your statement balance in full every month and the daily-interest math never touches you — the grace period keeps your effective interest at $0.
- If you must carry a balance, make a mid-cycle payment. Because interest is based on the average daily balance, paying $500 on day 10 instead of day 30 lowers the average balance for 20 extra days and shaves real dollars off the charge.
- Use the credit card interest calculator to see your exact monthly charge, and check the interest cost at your specific rate on the interest-by-APR pages before deciding how aggressively to pay down.
FAQ
Is credit card interest simple or compound?
Compound, and it compounds daily on most cards. Each day's interest is added to the balance, so the next day's interest is calculated on a slightly larger number. Over a month this compounding is modest, but over years of carrying a balance it is exactly why balances grow so fast.
How do I calculate my monthly interest by hand?
Multiply your balance by your APR, then divide by 12 for a quick monthly estimate. A $5,000 balance at 24% APR is 5,000 × 0.24 ÷ 12 = $100 for the month. For the precise figure, use the average daily balance × (APR ÷ 365) × days in the cycle.
Why is my interest charge higher than I expected?
Three common reasons: you lost the grace period by carrying a balance (so new purchases are charged from day one), you took a cash advance (higher APR, no grace period), or your average daily balance was higher than your statement balance because of mid-cycle purchases.
Does paying early reduce my interest?
Yes. Because interest is based on the average daily balance, every day a dollar is paid off early is a day it is not accruing interest. Paying a few days before the statement closes also lowers the balance reported to the credit bureaus, helping your utilization.
What APR will I actually pay?
It depends on your card and credit profile. The US average is roughly 21–24% in 2026, store cards often run 26–30%, and the best credit-union cards can be under 15%. See the interest-by-APR breakdown to compare what each rate costs on a real balance.