How credit card interest works, and how to pay less of it

Credit card interest is charged on any balance you do not clear by the due date. It is quoted as an annual percentage rate (APR) but usually calculated daily, and once you carry a balance it can apply to new purchases from the day you make them. Pay the full statement balance by the due date every month and, on most cards, purchases cost you nothing. This guide walks through the mechanics, and the credit card payoff calculator shows what your own balance will cost.

APR and the daily rate

Most issuers divide the APR by 365 (a few use 360) to get a daily periodic rate. At 24.9% APR that is about 0.0682% a day. On a 5,000 balance, that is roughly 3.41 of interest per day, or a little over 100 across a 30-day month. A card often has several rates at once: one for purchases, a usually higher one for cash withdrawals, and a promotional rate for balance transfers. In the US, a late payment can also trigger a higher penalty APR. Because interest is typically added to the balance each cycle and then charged interest itself, the effective annual cost is higher than the APR; APR vs APY: the rate before and after compounding explains that difference.

The interest-free period

The interest-free period (a "grace period" in the US) is why many people never pay card interest. If you pay the full statement balance by the due date, purchases on that statement are not charged interest. UK cards commonly advertise up to about 56 days interest-free on purchases, and in the US issuers that offer a grace period must send the statement at least 21 days before payment is due. The catch is that the period usually applies only when you clear the balance in full. Carry any amount over and you typically lose it: interest is then charged on new purchases from the day you make them until you are back to paying in full. Cash withdrawals are usually charged interest from the day you take them, plus a fee.

How a month's interest is worked out

Many issuers use the average daily balance: they add up the balance at the end of each day in the billing cycle, divide by the number of days, and apply the daily rate for each day. Suppose your balance is 2,000 for 15 days and then 3,000 for 15 days. The average daily balance is 2,500, and at 24.9% APR the month's interest is about 2,500 × 0.000682 × 30, or 51.16. This is why paying part of your bill early in the cycle, rather than on the due date, slightly reduces interest: it lowers the balance on more days.

The minimum payment trap

The minimum payment is usually a small percentage of the balance, or the month's interest plus about 1%, with a fixed floor. Because it shrinks as the balance shrinks, paying only the minimum stretches repayment out for years. In the calculator, a 5,000 balance at 24.9% with a minimum of 3% of the balance (floor 25) takes 251 months, almost 21 years, and costs about 9,946 in interest: nearly double the original debt. Try a 2% minimum and the calculator warns you that the balance never falls, because 2% is less than the monthly interest of about 2.08%. Real minimum formulas generally include the interest for exactly that reason.

A worked payoff example

Paying a fixed amount changes everything, because the payment no longer shrinks with the balance. On the same 5,000 at 24.9%:

• Paying 200 a month clears it in 36 months with about 2,123 in interest.
• Paying 250 a month clears it in 27 months with about 1,527 in interest, saving about 596 and nine months.
• Paying 100 a month never clears it: the first month's interest alone is about 104.

The calculator compounds monthly at APR/12, so real statements with daily interest, fees and new spending will differ slightly, but the shape is the same. Its month-by-month table makes the early months sobering: at 200 a month, more than half of the first payment is interest.

Ways to pay less interest

• Pay in full whenever you can, so the interest-free period keeps working.
• Pay a fixed amount above the minimum, and keep it fixed as the balance falls.
• Stop new spending on the card you are clearing; use a debit card meanwhile.
• Consider a 0% balance transfer, remembering the transfer fee and what the rate becomes when the offer ends.
• Compare a consolidation loan at a lower rate; the loan comparison calculator shows the total cost including fees.
• Clear the highest-rate card first if you have several; the debt snowball calculator compares that with the smallest-balance-first approach, and Debt snowball vs debt avalanche: which payoff method wins? explains the trade-off.

Avoid cash withdrawals on a credit card except in a genuine emergency, and if repayments feel unmanageable, a free debt-advice service can help before arrears build up.