A method and apparatus for performing calculations in a character input mode of an electronic device. Character input is received in the character input mode, and input characters are displayed in a character input window. If determined that an arithmetic expression is present in the displayed input characters, the arithmetic expression is evaluated, and an evaluation result is displayed. A calculation enabling condition may be satisfied prior to, or after, the arithmetic expression identification. The method may be applied to character input modes of a text messaging or email application, or of a word processing application.