Almost everything confusing about credit card interest comes from one gap: the number advertised on the card is an annual percentage rate, but interest is calculated daily. Once you see the daily arithmetic, the rest — why a balance grows faster than the APR suggests, why paying the statement in full is a cliff edge rather than a gradual benefit, why minimum payments barely move the balance — stops being mysterious.
The short version: APR ÷ 365 = your daily periodic rate. At 22.15% APR that's 0.0607% a day, applied to your average daily balance and compounded daily. Pay the statement balance in full and the grace period means you owe none of it. Carry any balance and you typically lose the grace period, so new purchases start accruing from the day they post. On $5,000 at 22.15%, a typical minimum payment takes about 19 years and costs more in interest than the original debt.
The daily periodic rate
Take the APR and divide by 365. That's the daily periodic rate — the number your issuer actually uses.
| APR | Daily periodic rate | Interest on $5,000 for one day |
|---|---|---|
| 18.00% | 0.0493% | $2.47 |
| 22.15% | 0.0607% | $3.03 |
| 25.16% | 0.0689% | $3.45 |
| 29.99% | 0.0822% | $4.11 |
Three dollars a day sounds survivable. The problem is that it compounds: tomorrow's interest is charged on today's balance plus today's interest. Over a 30-day cycle on a $5,000 balance at 22.15%, daily compounding produces $91.83 in interest rather than the $91.03 you'd get from flat simple interest. The 80-cent gap looks trivial in one month — but it's the mechanism that makes a long-carried balance so much more expensive than people expect.
Average daily balance — why mid-cycle payments help
Issuers don't charge interest on your closing balance. They charge it on your average daily balance: they record what you owe at the end of each day of the cycle, add those figures up, and divide by the number of days.
This has a practical consequence most people miss. Paying $500 on day 3 of a 30-day cycle reduces your average daily balance far more than paying the same $500 on day 28, because it's subtracted from 27 more daily readings. If you're carrying a balance and can't clear it, paying earlier in the cycle is strictly better than paying the same amount on the due date, even though both count as on-time.
The grace period is a cliff, not a slope
Federal rules require at least 21 days between your statement closing date and your payment due date. Pay the full statement balance inside that window and you're charged no interest on purchases at all — which is why someone who pays in full every month can hold a 29.99% card and genuinely never pay a cent of interest.
Two things about this surprise people:
- It only covers purchases. Cash advances almost always accrue interest from the transaction date with no grace period, and often at a higher APR plus an upfront fee. Balance transfers usually work the same way unless a promotional 0% rate applies.
- It's all or nothing. Pay $4,900 of a $5,000 statement and you don't get 98% of the benefit — you typically lose the grace period entirely. Interest is charged on the average daily balance for the cycle, and new purchases start accruing from the day they post rather than being protected. Most issuers only restore the grace period after you've paid in full for a cycle or two.
That second point is the single most expensive misunderstanding in consumer credit. The gap between paying in full and almost paying in full isn't proportional — it's a step change.
What the minimum payment actually costs
Minimum payments are usually structured as a percentage of the balance plus that month's interest — commonly around 1% of principal plus interest, with a floor of roughly $25 to $40. The structure is designed so the balance does eventually reach zero, but only just.
Here's $5,000 at 22.15% APR under different approaches:
| Approach | Time to clear | Interest paid |
|---|---|---|
| Minimum (1% + interest) | 231 months (19.3 yr) | $8,158.74 |
| Fixed $150/month | 53 months (4.4 yr) | $2,834.29 |
| Fixed $200/month | 34 months (2.8 yr) | $1,767.76 |
| Fixed $250/month | 26 months (2.2 yr) | $1,297.68 |
The first row is the headline: paying the minimum on a $5,000 balance costs more in interest than the original debt, and takes nearly two decades. The reason is that the minimum shrinks as the balance shrinks — the payment falls just as fast as the debt does, so progress decays rather than accelerating.
The contrast with the second row is the actionable part. Fixing your payment at $150 — not a dramatic sum, and less than the first minimum payment of about $142 plus a little — clears the same debt in 4.4 years instead of 19.3, and saves roughly $5,300 in interest. The single most effective thing you can do with a card balance isn't finding a lower rate; it's refusing to let the payment shrink.
Which "average APR" should you believe?
Published averages disagree a lot, and it's because they measure different things:
- 22.15% — the Federal Reserve's figure for May 2026, covering accounts actually assessed interest. This is the number that matters if you carry a balance, because it reflects rates real balance-carriers are paying.
- 25.16% — Forbes Advisor's average across the card offers in its database, i.e. advertised rates weighted by the cards it tracks.
- 19.35% — WalletHub's July 2026 figure, calculated across a different card mix.
None is wrong; they answer different questions. For working out what a balance will cost you, use the rate on your statement, not any average — the spread between a good credit-tier rate and a subprime one is wide enough to make averages close to meaningless for an individual.
What to do with this
- Pay in full or understand exactly what you're giving up. The grace period is binary. If you can clear the statement balance, the card's APR is irrelevant to you.
- If you're carrying a balance, fix the payment amount. Don't pay "the minimum" — pick a number and hold it as the balance falls. That one change does more than any rate shopping.
- Pay early in the cycle, not on the due date, if you're carrying a balance. Average daily balance rewards it.
- Treat cash advances as a separate, worse product. No grace period, higher rate, upfront fee.