Credit card interest is calculated daily using your APR divided by 365. Example: $5,000 balance at 22% APR = $5,000 × (0.22/365) = $3.01 daily interest. Over 30 days, that's $90+ added to your balance, which then compounds.
- Daily Periodic Rate (DPR) = APR ÷ 365
- Daily interest = Balance × DPR
- Interest compounds daily (interest on interest)
- Paying early in billing cycle reduces interest
- Grace period: 21-25 days if you pay in full each month
| Balance | APR | Monthly Interest | Annual Interest |
|---|---|---|---|
| $3,000 | 22% | ~$55 | ~$660 |
| $5,000 | 22% | ~$92 | ~$1,100 |
| $10,000 | 22% | ~$183 | ~$2,200 |
| $15,000 | 22% | ~$275 | ~$3,300 |
Credit cards use "average daily balance" method - your balance is averaged over the month, then interest is calculated. Paying mid-month reduces your average daily balance and thus your interest. This is why making multiple payments per month can save money.