A credit card statement shows one interest charge and none of the arithmetic behind it. The arithmetic is worth seeing once. It explains why the day you pay changes the bill, and why interest can still show up the month after you clear the card.
What follows is one card and one billing cycle, worked by hand, so you can redo it with your own statement.
From APR to a daily rate
Your card’s APR is a yearly rate, but most issuers charge interest by the day. They divide the APR by 365 (a few use 360) to get a daily periodic rate. A card with a 24.99% APR has a daily rate of 0.2499 divided by 365, or about 0.0685% per day.
That daily rate is applied to your balance. The most common method is the average daily balance, including new purchases. The issuer records your balance at the end of each day in the billing cycle, adds those daily figures together, and divides by the number of days. Interest for the cycle is that average, times the daily rate, times the number of days in the cycle.
The Consumer Financial Protection Bureau has a short explainer on how card companies calculate interest.
A worked example: one 30-day cycle
Take a card with a 24.99% APR and a 30-day billing cycle. The cardholder carried a $2,000 balance over from last month, so there is no grace period this cycle (more on that below). Three things happen:
- Days 1 to 10: the balance sits at $2,000.
- Day 11: a $600 purchase raises it to $2,600 for days 11 to 20.
- Day 21: a $500 payment brings it down to $2,100 for days 21 to 30.
Add up the daily balances: $2,000 for 10 days is $20,000, $2,600 for 10 days is $26,000, and $2,100 for 10 days is $21,000. The total is $67,000. Divide by 30 days and the average daily balance is $2,233.33.
Now apply the rate: $2,233.33 times (0.2499 divided by 365) times 30 days comes to $45.87 of interest for the cycle. If the same average balance sat on the card all year, the interest would be about $558, or roughly $46.51 a month. Many issuers also compound daily, adding each day’s interest to the balance; on this cycle that would raise the charge to about $46.33.
Why the day you pay matters
Because every day’s balance counts, a payment made early in the cycle lowers more days than the same payment made late. Keep everything in the example the same, but move the $500 payment from day 21 to day 3.
Now the balance is $2,000 for 2 days, $1,500 for 8 days, and $2,100 for the last 20 days after the purchase. That totals $4,000 plus $12,000 plus $42,000, or $58,000. The average daily balance drops to $1,933.33, and interest for the cycle falls to $39.71.
The saving is $6.16 for this one cycle. If the same pattern repeated every month, it would be about $74 over a year. Paying when your paycheck lands, or splitting a payment in two during the month, uses the same effect.
The grace period, and why some people pay $0
Most cards offer a grace period on purchases. If you pay the full statement balance by the due date, the issuer does not charge interest on the purchases from that statement. Federal rules require card issuers to mail or deliver your statement at least 21 days before the payment due date, and card agreements typically set the due date at least 21 days after the cycle closes. Card companies are not required to offer a grace period at all, though most do on purchases. The CFPB explains it in its page on what a grace period is.
In the example, the $600 purchase cost interest only because the cardholder brought a balance into the cycle. Someone who had paid last month’s statement in full and made the same $600 purchase would owe nothing on it, as long as they paid this statement in full by its due date too.
Carry any balance past the due date, though, and the grace period usually disappears. From then on, new purchases start collecting interest the day they post. Paying 95% of the statement does not keep it. Many issuers restore the grace period after you pay the statement balance in full for one or two cycles in a row. Policies vary, and your card agreement or the issuer can tell you which applies.
Residual interest: the charge after you pay in full
The example cycle ends with a statement balance of $2,145.87 ($2,100 plus the $45.87 of interest). The cardholder pays it in full on day 20 of the next cycle, before the due date. The next statement can still show interest.
That charge is often called residual or trailing interest. Since there was no grace period, interest kept building on the $2,145.87 from the statement closing date until the payment arrived. At the same daily rate, 20 days on $2,145.87 is about $29.38.
To clear a card completely, call the issuer or check the app for the current payoff amount, which includes interest accrued up to that day. Paying that figure, or paying the statement balance again the following month, usually finishes the job.
Cash advances and other balances follow different rules
A single card can carry several balances at different rates: purchases, cash advances, a balance transfer, maybe a promotional offer. Cash advances generally have no grace period, so interest starts on the day you take the cash, and they often carry a higher APR plus an upfront fee.
When you pay more than the minimum, federal rules require the issuer to apply the extra amount to the balance with the highest APR first. The minimum payment itself can go toward the lower-rate balances, which is one more reason to pay above the minimum when you carry a mix.
Where to find your card’s method
- The card agreement. The rate and fee table near the front lists your APRs and a line describing how the balance is calculated, for example average daily balance including new purchases.
- The paying interest section. It states how long your grace period lasts and when interest starts on purchases.
- Your monthly statement. The interest charge section lists each balance type, its APR, and the balance subject to interest rate for that cycle.
Where to put new spending
While a card carries a balance, every new purchase on it collects interest from the day it posts, at about 0.0685% a day on a 24.99% card. A $120 grocery run left there for a month adds roughly $2.50. Put day-to-day spending on debit, or on a card you pay in full, until the first card is cleared.
Related reading
- Personal Loan or Credit Card for a $6,000 Bill?
- Will Closing a Credit Card Hurt Your Score?
- Debt-to-Income Ratio: The Number Behind Many Loan Declines
Issuers differ in how they calculate balances and when a lost grace period comes back. Your cardholder agreement states the method your card uses.
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