How Credit Card Interest Is Actually Calculated
A 24.99% APR is really 0.06847% a day, compounded, on your average daily balance. Understanding that sentence is worth about $7 a month for free, and understanding the grace period is worth far more.
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- Read
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- 0.06847%
- Daily rate behind a 24.99% APR
- 14 years
- Minimum payments on $3,000
- $0
- Interest if you pay in full, always
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Credit card interest is not charged monthly on your statement balance, which is what almost everyone assumes. It is charged daily, on the average of every balance you held during the cycle, and it compounds. A 24.99% APR is a daily rate of 0.06847%, which on $3,000 across a 30-day cycle produces $62.27 rather than the $62.48 simple arithmetic suggests, and the mechanism matters far more than the 21 cents.
It matters because the same total payment produces different interest depending on when in the cycle you make it, because the grace period is all-or-nothing, and because four separate APRs can be running on one card simultaneously.
The daily periodic rate
Divide the APR by 365. That is the rate applied to your balance each day, and yesterday's interest is part of today's balance.
| APR | Daily rate | Interest for the cycle | Annual cost if held |
|---|---|---|---|
| 17.99% | 0.04929% | $44.53 | $591 |
| 21.49% | 0.05888% | $53.20 | $710 |
| 24.99% | 0.06847% | $62.27 | $836 |
| 27.99% | 0.07668% | $69.79 | $941 |
| 29.99% | 0.08216% | $74.83 | $1,010 |
Compounded daily. The annual column exceeds APR × balance for the same reason, daily compounding produces an effective annual rate above the nominal APR, roughly 28.4% effective on a 24.99% nominal.
The last column is the number to hold onto. Carrying $3,000 at a fairly ordinary 24.99% costs $836 a year, which is more than most people's annual saving from every discount, reward and negotiation they will pursue in the same period.
The average daily balance
The issuer records your balance at the end of each day, adds the thirty figures together, and divides by thirty. That average is what interest is charged on, which means the timing of your payments changes the bill even when the amount does not.
| Days | Balance | Days × balance |
|---|---|---|
| 1–9 | $2,000 | $18,000 |
| 10–19 (after an $800 purchase) | $2,800 | $28,000 |
| 20–30 (after a $500 payment) | $2,300 | $25,300 |
| Total | $71,300 | |
| Average daily balance | $2,376.67 | $71,300 ÷ 30 |
| Interest at 24.99% | $48.81 | $2,376.67 × 0.06847% × 30 |
The closing balance was $2,300 and the opening balance was $2,000, but interest was charged on neither. It was charged on $2,376.67.
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The grace period, and how it disappears
If you pay your statement balance in full by the due date, purchases in the next cycle are interest-free until that statement's due date. Commonly 21 to 25 days. This is why paying in full every month means never paying interest, regardless of how much you spend.
The part that catches people is that the grace period is not proportional. It is a switch.
| Pays $3,000 | Pays $2,900 | |
|---|---|---|
| Interest on the old balance | $0 | $2.08 on the $100 remaining |
| Grace period next cycle | Retained | Lost |
| New $1,200 of purchases | Interest-free | Accrues from the purchase date |
| Interest on new purchases | $0 | About $19 |
| Cost of the last $100 | — | About $21 |
The person who paid $2,900 kept $100 and paid roughly $21 for it. An effective rate on that $100 of over 250% annualised. This is the most expensive $100 in consumer finance.
Why minimum payments never end
A typical minimum is 1% of the balance plus that cycle's interest and fees, with a floor of $25 to $35. Read that structure carefully: the only part reducing your debt is the 1%.
| Component | Amount |
|---|---|
| Interest for the cycle | $62.27 |
| 1% of the balance | $30.00 |
| Minimum payment | $92.27 |
| Of which reduces the balance | $30.00 |
| Share of the payment that is interest | 67% |
| Monthly payment | Months to clear | Interest paid | Total paid |
|---|---|---|---|
| Minimum (1% plus interest) | 164 | $4,360 | $7,360 |
| $150 fixed | 26 | $915 | $3,915 |
| $250 fixed | 14 | $487 | $3,487 |
| $500 fixed | 7 | $235 | $3,235 |
The minimum-payment row runs for about fourteen years and pays $1,360 more in interest than the entire original balance. A fixed $150, barely more than the opening minimum of $92. Clears it in just over two years.
The important comparison there is the first two rows. The difference between them is $58 a month at the outset, and it is worth $3,445 and twelve years. Minimum payments are not a smaller version of paying the card off; they are a different financial product.
Four APRs on one card
Your card almost certainly has several rates running in parallel, and payments are not distributed across them the way you might expect.
| Balance type | Typical APR | Grace period | Fee |
|---|---|---|---|
| Purchases | 18%–28% | Yes, if paid in full | None |
| Balance transfers | 0% intro, then 18%–28% | No | 3%–5% |
| Cash advances | 25%–30% | Never | 3%–5%, minimum $10 |
| Penalty rate | Up to 29.99% | No | Plus a late fee |
Issuers must apply the portion of your payment above the minimum to the highest-APR balance first. The minimum itself can be applied to the lowest, which is why a low-rate balance can persist while you are paying more than the minimum every month.
The penalty rate is the one worth understanding before it applies. Trigger it, usually by going 60 days past due, and the issuer can raise the APR on your existing balance. Returning to a normal rate requires six consecutive on-time payments, and the issuer is only obliged to reconsider the rate on the existing balance at that point, not to remove it from new purchases.
Variable rates, and the part you can negotiate
Nearly all card APRs are variable and expressed as prime plus a margin. If prime is 7.50% and your card is prime + 17.49%, your APR is 24.99%. When the benchmark moves, your rate follows within a cycle or two, and the issuer does not need to notify you because the formula has not changed.
The margin is the negotiable part. It was set from your credit profile when the account opened, and if that profile has improved since, or if you have several years of on-time payments behind you, the issuer's retention department can revise it. Roughly half of people who ask get something, and the request costs a twelve-minute phone call.
What to check on your own statement this month
- The purchase APR, and whether it is quoted as prime plus a margin.
- The cash advance APR and fee. Usually several points higher.
- Whether you have a promotional rate, and its exact expiry date.
- The minimum payment formula, in the fine print rather than the amount due box.
- The statement closing date, which is what determines your reported utilisation.
- Whether an interest charge appears at all. If it does, the grace period is gone and everything you spend is accruing from day one.
Frequently asked questions
- If I pay in full every month, does my APR matter at all?
- For interest, no. The grace period means you never pay any. It still matters as insurance against a month when something goes wrong, and cash advances have no grace period regardless of how you pay.
- Does paying twice a month reduce interest?
- Yes, if you are carrying a balance, because it lowers the average daily balance. The saving is modest, a few dollars a month on typical balances, but it is free and requires no negotiation.
- Why did I get an interest charge after paying my balance in full?
- Residual or trailing interest: interest accrued between your statement closing date and the day your payment posted. It appears on the following statement and is normal. Pay it, and the account goes to zero.
- Can my issuer raise my rate whenever it likes?
- Not on an existing balance in the first year, and not without 45 days' notice for a fixed-rate increase on new purchases. Variable rates tracking a published index move freely, and the penalty rate applies once you are 60 days past due.
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