How Credit Card Interest Is Actually Calculated
Your APR is an annual number. Your card charges you daily. The gap between those two facts is why balances grow faster than people expect.
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Key takeaways
- Issuers divide your APR by 365 and apply it to your average daily balance.
- Daily compounding turns a 24.99% APR into roughly 28.4% effective over a year.
- Paying early in the cycle — not just on time — lowers the average daily balance and the interest.
Step 1: the daily periodic rate
Divide the APR by 365. At 24.99%, that's 0.06847% per day. It looks like nothing. It isn't.
Step 2: the average daily balance
The issuer adds up your balance on every day of the billing cycle and divides by the number of days. A big purchase on day 2 counts for 28 days; the same purchase on day 29 counts for one. Payments made mid-cycle lower the average, which is why paying early (not just on time) saves money.
Step 3: multiply
$5,000 × 0.06847% × 30 days ≈ $102.70
One month's interest on a $5,000 average daily balance at 24.99%. Most issuers compound daily, so the true figure is slightly higher.
Why "compounding daily" matters
Each day's interest is added to the balance before the next day's interest is calculated. Over a year, a 24.99% APR charged daily works out to roughly 28.4% effective. The card agreement will say "APR"; your statement reflects the compounded result.
What this means in practice
- Pay as early in the cycle as you can, not just by the due date.
- Split one payment into two (mid-cycle and due date) if cash flow allows. It lowers the average daily balance.
- Cash advances usually have a higher APR and no grace period. Their interest starts day one.
The one-line version
Your APR is annual. Your card charges you daily.
