Credit card interest is the most expensive borrowing most people ever pay for, and the mechanism behind it is not intuitive. This guide works through what actually happens on a statement, why minimum payments extend a balance for years, and what the payoff order changes.
The formula on your statement
Credit cards compound daily, and the standard calculation is:
Interest = average daily balance × (APR ÷ 365) × days in period
Two things follow immediately. The rate is applied daily, so the effective monthly rate is higher than APR ÷ 12 — and because interest is added to the balance, tomorrow's interest is charged on today's interest. That is compound interest at roughly 2% a month, which is the mechanism described in the simple versus compound guide.
Most issuers now use the average daily balance, which sums each day's balance and divides by the number of days. The practical consequence is that a payment made early in the cycle reduces the interest for the whole period, and a payment made on the last day barely helps that statement. When timing a payment is possible, the early slot is the better one — the credit card interest calculator will show how much.
What 24% APR means per month
APR is an annual figure. The daily rate is 24 ÷ 365 ≈ 0.0658%, and over a 30-day month:
(1.000658)³⁰ − 1 ≈ 2.0%
So 24% APR is about 2% a month, not 24%. Use the monthly figure when comparing against other borrowing: a credit card at 24% APR costs roughly 2% a month, whereas a personal loan at 12% APR over five years costs under 1% a month on amortising balance. The card is the expensive option, and the monthly comparison is what reveals it.
Worked example: the minimum payment trap
A $5,000 balance at 24% APR, paying the typical minimum of interest plus 1% of balance.
Month 1: interest ≈ 5,000 × 0.02 = $100. Minimum payment ≈ 100 + 50 = $150. New balance ≈ $4,950.
Month 12: balance ≈ $4,140, monthly interest ≈ $83.
Month 36: balance ≈ $2,505. The balance is falling — and you have paid roughly $5,400 in interest to get there, for $3,500 of principal repaid.
Continue to zero and the total interest approaches $6,100 on a $5,000 purchase, at 24% APR. The balance fell by half in three years, which is what a minimum payment is designed to do: keep the account current and profitable, not clear. The credit card payoff calculator shows the full trajectory and the total interest to a given date.
Why the balance falls so slowly
Because the minimum is structured as interest plus roughly 1% of principal. At 24% APR on a $5,000 balance, the interest alone is $100, so only $50 touches principal — 1% of what you owe. Even as the balance falls, the interest falls only slightly, and the principal reduction stays around 1%.
That structure has an exact consequence: time, not payment, is what kills these balances. Paying more than the minimum accelerates everything, because every extra dollar reduces the principal on which the next month's interest is charged. The general debt payoff calculator will show the same effect across loans.
Avoidance versus snowball
Two standard orderings, and they are genuinely different strategies rather than the same thing in different words.
Avalanche — highest interest rate first. Mathematically optimal. Total interest is minimised, because money repaid at 29% saves more than the same money repaid at 18%. If your goal is the lowest possible cost, this is the order.
Snowball — smallest balance first. Costs slightly more in total interest, but clears individual accounts faster. Each cleared balance removes a minimum payment obligation and frees cash, which accelerates the next payoff.
The evidence is fairly clear that behaviourally the snowball often wins, because a cleared card is a visible, reinforcing result, and the people who stick to a plan are the ones doing better overall. This is a case where the arithmetically optimal strategy is not the best strategy for the person running it — worth knowing rather than hiding, and the payoff calculators above will let you compare the two orderings on your actual numbers.
Practical things that reduce the cost
Pay in full every month. The card is then a payment tool costing nothing, and the interest mechanics never engage. If you are carrying a balance, this is the target state, and the budget calculator is where to find the monthly amount.
If you cannot pay in full, pay more than the minimum, consistently. The relationship between extra payment and time saved is not linear — $100 extra a month on a $5,000 balance at 24% removes roughly 2.5 years and about $2,000 of interest.
Order by rate, not by balance. Unless you are going to use the snowball approach deliberately for behavioural reasons, avalanche minimises the cost.
Check for a promotional balance transfer rate. A 0% transfer fee lasts longer than the promotion.
Watch the expiry date on any promotion. A 0% period ending with a 27% balance and no plan is worse than having paid it down steadily at 19%.
Consider a consolidation loan. A single lower-rate loan can clear several cards — but only if the freed monthly payment goes to repaying the new loan rather than back to the cards.
Do not add new balances to a card you are paying down. It is the most common reason a payoff plan stalls after three months.
The grace period, and why it is not what it sounds like
A full statement period of interest-free float sounds like free money. The detail is that the interest-free period applies to purchases, and it starts on the transaction date rather than the statement date. A large purchase late in the cycle gets almost no grace period, while the payment you make does not clear the interest until after the closing date. This is why the same balance carried across two statements can be charged twice, and it is worth checking the closing date on the statement if that has ever happened to you.
Balance transfers have their own rule, typically with a 3–5% transfer fee, and promotional 0% periods of 12–21 months are common. The arithmetic on a promotional rate is set out in the compound interest guide — the risk is entirely about what happens at the expiry date, and the way to manage that risk is to have the payoff amount already calculated before the promotion ends, not after.
Frequently asked questions
How is credit card interest calculated?
Typically (average daily balance) × (APR ÷ 365) × days in the period. It compounds daily, so interest is charged on interest. Most issuers use the average daily balance, which excludes days the balance was zero.
What does a 24% APR actually mean per month?
About 2% a month. The daily rate is APR ÷ 365 ≈ 0.0658%, and a 30-day month compounds to about 2.0%. That monthly figure is the right one to compare against other borrowing costs.
Why do minimum payments keep balances alive for years?
A minimum is roughly interest plus 1% of balance, so barely reduces principal. At that rate a balance falls slowly while interest continues, and the total cost can exceed the original purchase price.
Does payoff order matter?
Both orderings beat paying minimums on everything. Highest-rate-first (avalanche) costs less in total interest. Smallest-balance-first (snowball) costs slightly more but clears accounts faster, which is why it often works better in practice.