A credit card comparison interest calculator can estimate what you might repay by comparing the APR, balance, repayment amount and repayment period for different cards. Include any balance-transfer fees, annual fees and introductory offers, then check what rate applies when a promotional period ends. The result is an illustration rather than a guarantee, because the rate and credit limit offered depend on your circumstances and the lender’s assessment. Before applying, check the card’s full terms and consider FCA-authorised providers where possible.