How does credit card interest work?
Written byEmily Tye
Updated:Mar 20, 2026
5 min read
Credit card interest is the cost of carrying an unpaid balance. When you don't pay your full statement balance by the due date, interest is charged on what you owe. It builds up every day based on your annual interest rate and the size of your balance, and is added to your account at the end of each billing cycle.
Pay your balance in full each month and, on purchases, you pay no interest at all.
For an explanation of what APR is and how different rate types work, see our guide to what APR means on a credit card.
How is credit card interest calculated?
Credit card interest isn't charged as a single monthly figure. It builds up day by day, based on three things: your APR, your average daily balance, and the number of days in your billing cycle.
The daily rate
Your APR is an annual figure, so your lender converts it into a daily rate by dividing it by 365. If your card has an APR of 24%, your daily rate is:
24 ÷ 365 = 0.066% per day
Your average daily balance
Rather than looking at what you owe on one specific day, your lender tracks your balance every day throughout the billing cycle, adds all those figures together, and divides by the number of days.
So if you owe £1,000 for the first 15 days of a 30-day cycle, then spend £500 more and owe £1,500 for the remaining 15 days, your average daily balance is £1,250.
The full calculation
At the end of the billing cycle, your interest charge is:
Average daily balance × daily rate × days in the cycle
Using the example above, with a 24% APR over 30 days:
£1,250 × 0.00066 × 30 = £24.75
That's the interest added to your next statement.
This is a simplified illustration using a fixed balance and a round APR figure. In practice, your balance will change throughout the month as you spend and make payments, and your card's APR may differ. Your actual interest charge will vary.
When does interest start being charged?
The grace period
Most credit cards include a grace period (the window between your statement date and your payment due date). If you pay your full statement balance by the due date, no interest is charged on purchases at all.
Because of how billing cycles work, this means most UK credit cards offer up to 56 days interest-free on purchases. A purchase made at the start of your billing cycle has the rest of that cycle plus the grace period before interest kicks in. A purchase made near the end of your cycle has less time.
The key word is "full." Pay anything less than your complete statement balance, even by a few pounds, and the grace period doesn't apply. Interest then accrues from the date each purchase was made.
Cash advances are different
If you use your credit card to withdraw cash, interest starts building from the moment of the transaction. There's no grace period, and the rate is typically higher than the standard purchase rate. See our guide to withdrawing cash on a credit card for more.
How does compounding work on a credit card?
Compounding means that unpaid interest gets added to your balance, and then you're charged interest on that interest in the following month.
Here's a simplified illustration. Say you owe £1,000 and carry it for three months at a 24% APR, without making any payments:
Month 1: Balance £1,000. Interest charged: roughly £19.73. New balance: £1,019.73.
Month 2: Balance £1,019.73. Interest charged: roughly £20.12. New balance: £1,039.85.
Month 3: Balance £1,039.85. Interest charged: roughly £20.51. New balance: £1,060.36.
This is a simplified illustration. It assumes no new spending and no payments during the period. Real balances change throughout the month, which affects the calculation.
The individual amounts look modest at first. But over many months, this effect becomes significant. The longer a balance runs, the faster the total grows.
Does interest work the same way for all transactions?
The calculation is the same, but when interest starts and what rate applies can differ.
Purchases: Interest only applies if you carry a balance past your due date. Pay in full each month and no interest is charged.
Cash advances: Interest starts immediately, with no grace period, at a rate that's typically higher than the purchase rate.
Balance transfers: Many cards offer a 0% introductory period on transferred balances. During that window, no interest applies to the transferred amount. Once the promotional period ends, the standard rate takes over on anything remaining. It's worth checking your card's terms before that date arrives.
How to keep your interest charges down
The most reliable way to avoid credit card interest is to pay your full statement balance by the due date every month. Do that consistently and you're effectively using the card interest-free on purchases.
"If you pay your full statement balance every month, the rate becomes almost irrelevant for purchases. The daily compounding effect only works against you if you let a balance run. Understanding that is the most useful thing you can know about how a credit card actually costs you money."
5 years in financial services
If you can't clear the full balance, paying as much as possible still reduces the cost. Interest is charged on your remaining balance, so the less you carry over, the less you'll pay.
Avoid using your card for cash withdrawals unless necessary. The higher rate and immediate interest accrual make them the most expensive way to borrow on a credit card.
If you're carrying a balance and working out how to pay it down, our guide to paying off credit card debt covers the practical options.
FAQs
This blog is for informational purposes only and does not constitute financial advice. Please speak to a qualified financial adviser before making financial decisions.
