18 yuan. Suppose you overdraw your credit card balance by 1,, and the annual interest rate of the credit card is 18%. The interest calculation formula is: interest = overdraft amount × annual interest rate = 1,× .18 = 1,8 yuan. So, if you overdraw the credit card balance of 1, yuan and calculate the interest for one year at the annual interest rate of 18%, the interest amount is 1,8 yuan.