How credit card interest really works — and what it is costing you

Almost everyone knows that carrying a credit card balance is expensive. But very few people understand exactly how credit card interest is calculated — and that lack of understanding often leads to decisions that cost more than they realise.

What APR actually means

APR stands for Annual Percentage Rate. It is the interest rate you are charged on your outstanding balance over the course of a year. A card with 24% APR charges 24% of your balance in interest per year — but credit card interest is usually calculated and charged monthly, which means the actual mechanics are slightly more complex than a single annual number suggests.

How daily periodic rate works

Most credit card issuers calculate interest using a daily periodic rate — which is your APR divided by 365. So a card with 24% APR has a daily rate of roughly 0.0658%. This rate is applied to your average daily balance over the billing cycle, not just your balance on a single day.

This means that every day you carry a balance, interest is accruing — including on purchases you made partway through the billing period.

The minimum payment trap

Credit card minimum payments are typically set low — often 1-3% of the balance or a small fixed amount, whichever is greater. Paying only the minimum means the majority of your payment goes towards interest rather than reducing the principal balance. It can take years — sometimes decades — to pay off a significant balance by making only minimum payments, and the total interest paid can exceed the original amount borrowed.

Your credit card statement is required to show you how long it will take to pay off your balance making only minimum payments, and the total interest cost. If you have not looked at that section before, it can be a sobering read.

Grace periods — and when they disappear

If you pay your full statement balance by the due date each month, most credit cards charge you no interest at all on purchases — this is the grace period. The moment you carry a balance, the grace period typically disappears, and interest starts accruing on new purchases from the day they are made, not just on the carried balance.

The fastest way to reduce what you pay in interest

The most effective way to reduce credit card interest costs is to pay more than the minimum — ideally as much as you can afford each month. Even paying a fixed amount above the minimum makes a significant difference to the time it takes to clear the balance and the total interest paid.

If you carry balances across multiple cards, focusing your extra payments on the highest-rate card first (while paying minimums on others) reduces interest costs most efficiently.