Your card doesn’t charge interest once a year. Most issuers charge it every day, based on what you owe that day. Here’s exactly how the math works, with a real example you can follow.
The Formula in One Line
- Daily periodic rate: your APR divided by 365 or 360, depending on the issuer (CFPB).
- Average daily balance: add up each day’s balance, then divide by the number of days.
- Days in cycle: the length of your billing period, shown on your statement.
Step-by-Step Example
A 30-day billing cycle with a 22.36% APR, the average rate on cards that charged interest in August 2026, per Federal Reserve data. You’re already carrying a balance, so there’s no grace period.
Your balance during the 30-day cycle
Purchase on day 11, payment on day 21
What this chart shows
- Days 1–10: you owe $2,000.
- Days 11–20: a $500 purchase raises it to $2,500.
- Days 21–30: an $800 payment drops it to $1,700.
- The dashed line is your average daily balance: $2,066.67. Interest is charged on this number, not on your final balance.
- Daily periodic rate22.36% ÷ 365 = 0.06126%
- Average daily balance$2,066.67
- ($2,000 × 10 + $2,500 × 10 + $1,700 × 10) ÷ 30
- Days in billing cycle30
- Interest charged$37.98
Why your bill may differ by a few cents: many issuers compound daily, adding each day’s interest to the balance. Some divide by 360 instead of 365. Your card agreement says which method yours uses.
The Grace Period: How to Pay $0 Interest
A grace period is the time between the end of your billing cycle and your payment due date. Your issuer must send your bill at least 21 days before payment is due.
Pay the full statement balance
- Statement closes
- You pay the full balance by the due date
- New purchases stay interest-free
Interest charged: $0
Pay less than the full balance
- Statement closes
- You pay part of it (or just the minimum)
- You lose the grace period: interest hits the unpaid balance and new purchases from day one
Interest charged every day
- Issuers aren’t required to offer a grace period, but most do for purchases.
- Cash advances usually start charging interest right away, with no grace period.
- Federal rules ban double-cycle billing, so you can’t be charged interest on balances from earlier cycles after you lose the grace period.
What a Balance Really Costs
| Balance carried | Interest per 30 days | Interest per year* |
|---|---|---|
| $1,000 | $18.38 | $224 |
| $3,000 | $55.13 | $671 |
| $5,000 | $91.89 | $1,118 |
| $10,000 | $183.78 | $2,236 |
*At 22.36% APR, without daily compounding and assuming the balance stays the same. Actual costs are slightly higher with daily compounding.
- Every $1,000 you carry costs about $0.61 a day.
- Paying early in the cycle lowers your average daily balance, and your interest.
4 Ways to Pay Less Interest
Keeps your grace period, so purchases cost $0 in interest.
A payment mid-cycle lowers your average daily balance right away.
Much of a minimum payment can go to interest, so the balance shrinks slowly.
They usually charge interest from day one, often at a higher APR.
Frequently Asked Questions
Why was I charged interest after paying in full?
If you carried a balance the month before, you may owe interest for the days before your payment posted, sometimes called residual interest. Pay in full again next month to get your grace period back.
Is credit card interest compounded daily?
Often, yes. Many issuers add each day’s interest to the balance, so tomorrow’s interest is charged on today’s interest too.
Where do I find my APR and daily rate?
On your monthly statement, in the interest charge section, and in your cardholder agreement.
Does paying twice a month lower interest?
It can. An extra payment early in the cycle lowers your average daily balance, which lowers the interest charged.
How long until your card is paid off?
Enter your balance, APR, and payment. See your payoff date and total interest.
Sources
- CFPB: What is a daily periodic rate on a credit card?
- CFPB: How does my credit card company calculate the amount of interest I owe?
- CFPB: What is a grace period for a credit card?
- CFPB: Regulation Z §1026.54, Limitations on the imposition of finance charges
- FRED (Federal Reserve G.19): Interest rate on credit card plans, accounts assessed interest
This article is for educational purposes only and is not financial, tax, or legal advice. Rates and rules change, so confirm with your bank, lender, or a qualified professional before you decide.