Credit Card Calculator

How Credit Card Interest Really Works

A credit card is an unsecured revolving loan: the issuer lets you spend up to a credit limit, sends a monthly statement, and lets you repay anything from a small minimum to the full balance. Whatever is left unpaid rolls into next month and accrues interest at the card's APR — an annual percentage rate that, for cards, is high by loan standards, commonly 18–28%, because there is no collateral behind it. Pay the statement balance in full every month and the grace period means you never pay interest at all; carry a balance and the grace period usually disappears, so even new purchases start accruing immediately.

How Issuers Charge It: Average Daily Balance

Most issuers use the average daily balance method. The APR is divided by 365 to get a daily periodic rate; that rate is applied to the average of the balance on each day of the billing cycle, multiplied by the number of days. Paying earlier in the cycle, or making two smaller payments instead of one, lowers the average balance and therefore the interest — an effect this calculator's monthly model (APR ÷ 12 on the opening balance) approximates closely. Two older methods, previous balance and adjusted balance, still exist in some card agreements and produce lower charges, which is one reason statement interest rarely matches a simple estimate to the exact cent.

Cash Advances and Balance Transfers

Two features deserve special caution. A cash advance — withdrawing cash against the card — usually carries a higher APR than purchases, a fee of 3–5%, and no grace period: interest starts the moment the cash leaves the ATM. A balance transfer moves debt to a card offering a low or 0% introductory APR, typically for 6–21 months, usually for a 3–5% fee on the amount moved. It is a genuinely useful tool when the fee is smaller than the interest avoided and the balance is cleared inside the promotional window; left longer, the remaining balance reverts to the full APR and the trap closes again.

Using a Card Without Being Used by It

The same product that produces five-figure interest bills also offers real advantages when the balance is paid in full: fraud liability capped by law (and usually zero by policy), purchase protection and extended warranties, cashback or travel rewards that amount to a small discount on everything, and a documented payment history that builds a credit score. The dividing line is never the card — it is whether a balance revolves. If one already does, the fastest exit is a fixed payment as large as the budget allows, aimed at the highest APR first (the "avalanche" order), while new spending goes on debit until the balance is gone.

Frequently Asked Questions

Why doesn't my statement interest match this exactly?

Issuers use daily interest on the average daily balance, and your payment dates and mid-cycle purchases move the average. Expect agreement within a few cents per month, not to the penny over years.

What if I only pay the interest each month?

The balance never moves: $120 a month on the default balance pays the issuer $1,440 a year and reduces your debt by $0. Any amount above the interest starts making progress.

Is "interest + 1%" a sensible payment?

It is the standard minimum formula, built to stretch the debt. Use the presets above to see its first-month amount, then compare holding that amount fixed against choosing a deadline — the fixed plan is always faster.

When does a balance transfer pay off?

When the transfer fee (often 3–5% once) is less than the interest you'd pay over the same months, and you can clear the balance before the promotional APR expires.