Most credit card companies require a minimum monthly payment of between 2% and 4% of the account balance. For example, if your card issuer requires 3%, and you owe $800 this month, your minimum payment would be $24 ($800 x .03 = $24). To figure out how much of that is going to interest charges, divide your interest rate by 12 (months) to get the periodic (monthly) interest rate. Let’s say you’re paying 20% interest, your periodic rate would be .016666% (.20/12=.016666). Then multiple that rate by your outstanding balance (.016666 x $800 =$13.33) and you find out that $13.33 of your $24 payment is going toward interest and only $10.67 is going toward reducing your balance.
Bankrate offers an online credit card calculator.