How credit card interest works

The grace period is the whole game

Before any of the arithmetic below matters, one rule decides whether you pay interest at all.

If you pay your statement balance in full by the due date, most cards charge no interest on purchases. That window between the statement closing and the payment being due is the grace period, and the CARD Act requires issuers who offer one to give you at least 21 days.

Pay in full, and your APR is a number you never encounter. Everything that follows applies only once you carry a balance.

The daily periodic rate

Most US issuers do not charge interest monthly. They charge it daily.

Daily periodic rate = APR ÷ 365

At 21.99% APR, that is 0.06025% a day. Each day, the issuer applies that rate to your balance for that day, and adds the result.

Average daily balance

Because your balance changes during a cycle, issuers compute an average. For each day, they take the balance at the end of the day; at the end of the cycle they average those figures and apply the periodic rate to the average, multiplied by the number of days.

Interest = average daily balance × daily periodic rate × days in cycle

That has a practical consequence: when you pay matters, not just whether you pay. A $500 payment made on day 3 of a 30-day cycle reduces 27 days of balance. The same payment on day 28 reduces two. Both clear the same amount of debt; the first costs less interest.

The credit card interest calculator models this: switch accrual between daily and monthly on the same APR and payment, and you can see how much of the total is the compounding rather than the rate.

A worked example

$3,000 at 21.99% APR, paying $150 a month, no new spending:

• First month's interest: roughly $55, charged before any of your $150 touches the principal.
• Principal repaid in month one: roughly $95.
• Time to clear: around 25 months.
• Total interest: roughly $700.

Raise the payment to $250 and it clears in about 14 months for roughly $380 of interest. Nearly half the interest disappears for one extra year of higher payments — which is the shape of every carried balance.

The different APRs on one card

A single card carries several rates, and they apply to different things:

Cash advances are the one to watch. There is typically a fee of 3–5% and interest from day one and a higher rate than purchases. Using a card at an ATM is one of the most expensive ways to borrow small amounts.

Promotional 0% offers

A 0% introductory APR is real, and it is time-limited. Two things to check before relying on one:

When does it end, and what is the go-to rate? The date is in the offer summary. Anything left on the balance when the promotion expires starts accruing at the standard rate.

Is it deferred interest? On some store financing — less common on general-purpose cards — interest accrues in the background during the promotional period and is charged retroactively in full if any balance remains at the end. That is materially different from a true 0% offer, where only the remaining balance starts accruing. The disclosure will say which.

Balance transfers usually carry a fee of around 3–5% of the amount moved, charged upfront. On a large balance that fee is real money, and it has to be weighed against the interest avoided.

What to do about it

1. Pay in full if you possibly can. It makes your APR irrelevant.
2. If you cannot, pay as early in the cycle as you can. Average daily balance rewards it.
3. Never treat the minimum as the target. See minimum payments.
4. Deal with the highest-rate balance first if you have several — the debt payoff calculator will show you the difference in dollars.