Credit card interest has a reputation for being complicated. It isn't, really. It's just explained badly, usually in the small print, usually after you've already run up the balance. Here's how it actually works.
Interest is charged daily, not monthly
The APR on your card is a yearly number, but the card doesn't wait a year to charge you, and it doesn't even wait a month. Interest accrues daily. The card takes your APR, divides it by 365 to get a daily rate, and applies that to your balance each day.
So a 22% APR is really a daily charge of 22% divided by 365, ticking away on whatever you owe that day. This matters because it means your balance isn't a static number that gets billed once a month. It's a running total that grows a little every single day you carry it. Pay a chunk off mid-cycle and you genuinely save money, because there are fewer days of interest on the higher balance.
The grace period: the free lunch that's actually real
Here's the part that makes credit cards genuinely great when used well. If you pay your statement balance in full within the grace period, you pay zero interest. Not reduced interest. Zero. The daily accrual I just described gets waived entirely for purchases when the statement is cleared on time.
That's a short-term interest-free loan, every month, forever, for the price of paying your bill. I'd go as far as saying this is the only way to use a credit card where the maths is unambiguously on your side.
The catch is the cliff edge. Carry even part of the balance past the due date and the grace period typically goes away, and interest starts accruing on purchases from day one. The gap between "pays in full" and "carries a small balance" is much bigger than it looks.
Minimum payments: designed not to work
Now the other side of the cliff. Minimum payments are typically about 2% of the balance, with a floor around $25. Two percent sounds like progress. It isn't, and one worked example shows why.
Take an $8,000 balance at 22% APR. The first minimum payment is about $160, which is 2% of $8,000. Of that $160, $146.67 is interest. That leaves $13.33 actually reducing what you owe. You paid $160 and your debt went down by about the price of a takeaway.
It gets worse in a sneaky way. As the balance falls, the minimum payment falls with it, because it's a percentage. So the payment shrinks to match your progress, and the payoff stretches out further and further. Minimum payments never realistically clear the balance. That's not an accident of the maths. A balance that never clears is a customer who pays interest forever, and the minimum is calibrated to keep you exactly there.
The fix: a fixed payment
The escape is almost stupidly simple: pick an amount and pay it every month regardless of what the statement asks for. When the payment stops shrinking, every month more of it hits the principal, and the whole thing snowballs in your favour instead of the bank's.
Same $8,000 at 22% APR: a fixed $300 a month clears it in 37 months, with $3,082.84 of total interest paid. That's still a lot of interest, and I won't pretend otherwise. But it's a debt with an actual end date, three years out, instead of one that trails on indefinitely while you post minimums into it.
Run your own balance through the credit card payoff calculator and try a few fixed amounts. Watching the payoff date jump forward as you nudge the payment up is the most motivating thing I can offer you. If you're juggling several debts and want to see how a fixed payment plays out across them, the loan payoff calculator does the same job for loans generally.
What I'd actually do
Three rules cover nearly everything. If you can pay the statement in full, always do it, and the card costs you nothing. If you can't, set a fixed payment as high as you can genuinely sustain, and treat the minimum as a trap rather than a suggestion. And if you're carrying a balance at anything like 22%, clearing it beats almost any saving or investment you could make with the same money, because a rate you stop paying is a return you can't lose.
The card doesn't care which kind of customer you are. The daily interest maths runs either way. You just get to pick which side of it you're on.