How credit cards work

💳 Credit Cards · 4 min read · Last reviewed 2026-10-02 · By the AlfaFinances editorial team

Each month, your credit card issuer totals up everything you spent into a statement and gives you a due date to pay it. Pay the full statement balance by that date and you typically pay zero interest on those purchases, this grace period is one of the most valuable features of a credit card, used correctly. Pay less than the full balance, and interest starts being charged on whatever's left, calculated based on the card's APR (annual percentage rate).

A worked example

Say your statement shows $800 spent this cycle, due by the 20th. If you pay the full $800 by the 20th, you owe nothing extra. If you pay $400 and carry $400 forward, that $400 starts accruing daily interest based on your APR (commonly 20%+ on standard cards), and importantly, many issuers also lose the grace period on new purchases too once you're carrying a balance, so it's not just the $400 that starts costing you.

Key takeaways

Paying the statement balance in full each month is what makes a credit card genuinely free to use.
Carrying a balance means real, ongoing interest charges, calculated daily against the card's APR.
Many cards remove the interest-free grace period on new spending too, once you're carrying any balance forward.

Common mistakes

Only paying the minimum payment shown on the statement, assuming that's "enough."
Not realizing new purchases can also start accruing interest immediately once a balance is being carried.
Missing the due date entirely, which can trigger both interest and a late fee.

Test yourself

1. What happens if you pay your full statement balance by the due date?

You still pay interest
You typically pay zero interest on those purchases
Your card gets cancelled
You get a penalty

Answer: You typically pay zero interest on those purchases. Paying in full by the due date is what keeps a credit card interest-free.

2. What triggers interest charges on a credit card?

Using the card at all
Carrying an unpaid balance past the due date
Having a high credit limit
Signing up for the card

Answer: Carrying an unpaid balance past the due date. Interest is charged on balances not paid off in full by the due date, based on the card's APR.

3. Can carrying a balance affect new purchases too?

No, new purchases are always separate
Yes, many issuers remove the grace period on new spending once any balance is carried
Only if you call to ask
New purchases are always free

Answer: Yes, many issuers remove the grace period on new spending once any balance is carried. Once a balance is carried forward, many cards start charging interest on new purchases immediately too, not just the old balance.

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Sources and further reading

Educational information only, not financial advice. AlfaFinances does not execute trades, hold your money or recommend that you buy or sell any security. Prices and company data come from public sources and may be delayed or inaccurate. Past performance does not indicate future results, and investing involves risk, including loss of principal. See our editorial policy and privacy policy. Last updated: 2026-10-02.