Credit Card Interest Calculator
Card interest accrues daily, not monthly. See what your balance costs per day, per billing cycle, and why the timing of a payment inside the cycle matters.
Interest that accrues while you sleep
Credit-card interest is not a monthly event; it is a daily drip. The daily periodic rate — APR divided by 365 — is applied to the balance every single day, and the cycle's interest is the sum of those drips. Seeing the per-day price of a balance changes how the balance feels.
Daily interest = balance × APR ÷ 365 · Cycle interest = daily interest × days in cycle
Example: $5,000 at 24% APR accrues $3.29 a day — $98.63 across a 30-day cycle, $1,200 across a year if the balance stood still. A $1,000 payment made on day 2 instead of day 28 saves about $17 of interest in that cycle alone.
The grace period is the real product
Pay the statement balance in full and most cards charge no purchase interest at all — the daily machinery only engages once you revolve. The distance between 'uses a card' and 'pays for a card' is entirely whether this page's numbers ever apply to you.
How this is calculated
Daily interest = balance × APR ÷ 365 (the issuer's daily periodic rate applied to a constant balance). Cycle interest multiplies by the days you enter. This isolates the accrual arithmetic; the full payoff simulation — where the balance, and therefore the daily drip, shrinks monthly — lives on the payoff calculator using the same model.
Frequently asked questions
Why do issuers use daily balance instead of monthly?
Because it prices accuracy to the day: a balance carried for 12 days costs 12 days of interest. It also means payment timing inside the cycle has real value, as the example shows.
Is the APR divided by 365 or 360?
Card issuers use 365 for the daily periodic rate under US practice (Regulation Z permits the disclosed method; 365 is standard for cards). Some commercial lending uses 360 — read your cardholder agreement's exact words.
Do new purchases accrue interest immediately?
If you are revolving (carrying a balance), usually yes — the grace period is lost. If you pay in full monthly, purchases typically get the grace period. Cash advances almost never have one, on any card.
How does a penalty APR change this?
A missed payment can trigger a penalty APR near 30% on many cards. Re-run the page at that rate to see why issuers' reminders about due dates are so cheerful and frequent.
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