How credit card interest really works

A credit card is two different loans wearing the same piece of plastic. Pay the statement balance in full every month and it is a rolling interest-free loan of up to eight weeks, usually with rewards on top. Carry a balance — any balance — and it becomes one of the most expensive debts a household can hold, with mechanics arranged so that the cost stays easy to underestimate.

Those mechanics reward being understood precisely. Daily accrual, a grace period that vanishes retroactively, a minimum payment sized to move almost nothing: each is disclosed somewhere in the cardholder agreement, none is intuitive, and together they explain how a mid-size balance can quietly cost more in interest than the purchases were worth. This guide walks through each one with real numbers.

Interest accrues every day, not once a month

The rate on your statement is annual — call it 20%, typical for a Canadian rewards card — but nothing about the charge is annual. The issuer divides the rate by 365 to get a daily rate of about 0.055%, then applies it to your average daily balance across the billing cycle. On a $6,000 balance that is about $3.29 a day, roughly $99 over a 30-day cycle.

Two consequences follow. First, timing inside the cycle matters: because interest is computed on the average balance, a payment that lands early in the cycle saves nearly a month more interest than the same payment made just before the statement closes. Second, the interest posted each month joins the balance and starts accruing interest itself, so the true annual cost of a 20% card is closer to 22%. This is the same compounding that builds wealth in your favour inside an investment account, running in reverse — at a rate no diversified portfolio reliably matches, which is the entire logic of paying off cards before investing.

The grace period is conditional — and it dies retroactively

Purchases are interest-free during the grace period, which Canadian issuers must set at 21 days or more from the statement date. The condition is paying the full statement balance by the due date, and it is all-or-nothing. Pay $5,900 of a $6,000 statement and you are not charged interest on the missing $100 — you are charged interest on the full average daily balance, backdated to when each purchase posted, as though the grace period had never existed. Nearly paying in full buys you nearly nothing.

Carrying a balance also suspends grace on everything new. While any amount rolls over, each fresh purchase — groceries, a coffee — starts accruing the day it posts, so the card's headline benefit is switched off at exactly the moment it would help. And there is a final sting on the way out: pay the "current balance" from your statement and a small charge often appears the next month anyway. That is residual interest, accrued between the statement date and the day your payment landed. Clearing a card completely means paying in full two cycles running, or phoning the issuer for a same-day payout figure.

Minimum payments are engineered to last decades

A typical Canadian minimum is around 3% of the balance, with a small floor. The formula is not designed to retire the debt; it is designed to cover the month's interest with a little left over, so the balance technically declines while staying alive as long as possible.

Watch it work on $6,000 at 20%. The first month's interest is $100, and 3% of the balance works out to $183 — so $83 goes to principal. Next month the minimum is 3% of a slightly smaller balance, so the payment shrinks too. The payment follows the balance down, always doing just better than keeping pace, and the payoff stretches across decades. The escape costs nothing except refusing to let the payment shrink:

Read the middle row again. The entire difference between 21 years and four — and between $6,984 in interest and $2,754 — comes from holding the very first minimum payment flat instead of letting it decay with the balance. Same card, same rate, same starting payment. The disclosure box on your statement showing "time to repay if you make only the minimum payment" is describing the first row; the second is available to anyone who treats the minimum as a floor rather than an instruction.

Run your numbersMinimum payment trap→

Minimum only$183 held fixed

One balance, two payoff paths

	Minimum only	$183 held fixed
0	6000	6000
5 yrs	3074	0
10 yrs	1128	0
15 yrs	578	0
20 yrs	143	0

$6,000 balance at 20% annual interest; minimum is 3% of the balance with a $10 floor. Both paths start at the same $183 payment.

Cash advances play by worse rules

Every protection above has a carve-out, and the cash advance sits in all of them at once. There is no grace period, ever — interest starts the moment the cash leaves the machine, regardless of how you handle your statements. The rate is usually several points above the purchase rate. And there is an upfront fee on top, commonly 3.5% with a minimum charge.

The numbers are small enough to feel harmless and priced like nothing else you borrow. Take $400 from an ATM on your card, repay it 20 days later: a $14 fee plus about $5 of interest at a typical advance rate makes $19 — for a 20-day loan of $400, an annualized cost of roughly 87%. The trap widens because "cash advance" is a category, not a button: lottery tickets, casino chips, cryptocurrency purchases and some person-to-person transfers are coded as quasi-cash and get advance treatment without the word ever appearing. Payment allocation adds a last quiet cost — the portion of a payment above the minimum must go to your highest-rate balance first (or be spread proportionally), but the minimum itself can be pointed wherever the issuer likes, which is rarely the advance.

Promotional rates do their work in the fine print

A balance-transfer promotion — a low rate for a fixed window, for a fee of 1% to 3% of the amount moved — is genuinely useful arithmetic. Moving the $6,000 above for a 3% fee costs $180, less than two months of interest at 20%. If the window is used to attack principal, the transfer pays for itself almost immediately; whether consolidation is worth it generally turns on exactly this trade of fee against rate.

The fine print is where promotions earn their keep for the issuer. The promo rate covers the transferred balance only — new purchases usually accrue at the full rate, often with no grace period while the promo balance sits there, so a transfer card should see no new spending at all. The window is hard-edged: whatever remains when it closes reverts to the regular rate, sometimes a higher one, so the payment that makes sense is the balance divided by the number of promo months, not the minimum. A single missed payment can void the promotional rate outright. And the harshest variant is deferred interest — store financing marketed as "no interest for twelve months" on furniture or electronics, where interest quietly accrues from day one and is charged retroactively on the whole original amount if any balance remains at the end. Owing $50 in the final month can trigger hundreds of dollars of backdated interest.

What actually gets you out

Three moves, in order. Stop adding to the card first: grace stays dead while a balance rolls, so every new purchase accrues from day one — run day-to-day spending through debit until the card reads zero. Second, fix the payment: pick a number comfortably above the current minimum and hold it flat, because the table above shows the shrinking payment is what converts a manageable balance into a decades-long one. Third, if you hold several debts, point the extra at the highest rate — the arithmetic, and the case for sometimes ignoring it, is the subject of avalanche versus snowball, and the debt payoff planner will sequence your actual balances. To see what your own card costs while you decide, the credit card interest calculator turns your balance and rate into a per-day figure, which tends to motivate in a way an annual percentage never does.

The card was never the problem; the balance is. The moment it reaches zero and one full statement is paid on time, the grace period comes back and the same piece of plastic quietly turns back into the interest-free product it pretends to be for everyone.