How Credit Card Interest Really Works

Credit cards are the most expensive common debt — and the way they charge interest makes balances grow faster than people expect. Here is what is actually happening.

APR and the daily rate

Your card's APR is the yearly rate, but interest is charged daily. The card divides your APR by 365 to get a daily rate, then applies it to your balance every single day. At 22% APR, that is about 0.06% per day — which compounds.

Average daily balance

Most issuers calculate interest on your average daily balance: they add up your balance for each day of the billing cycle and divide by the number of days. Carrying a balance even part of the month still accrues interest.

The grace period: your interest-free loan

If you pay your statement balance in full every month, most cards give you a grace period and charge zero interest on purchases. This is the single most powerful trick: pay in full and the card becomes a free short-term loan.

The moment you carry a balance, you typically lose the grace period — and new purchases can start accruing interest immediately.

Why a balance snowballs

Because interest compounds daily and is added to your balance, you start paying interest on your interest. Combined with low minimum payments (often just 1–3% of the balance), this is how a few thousand dollars can take a decade to pay off.

How to beat it

Pay in full whenever possible to use the grace period. If you are carrying a balance, attack the highest-APR card first (the avalanche), consider a 0% balance transfer, and pay far more than the minimum. Model your payoff date here to see how much faster extra payments get you there.

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