Junpei obtains this tool early on, at the beginning of the game in the 3rd class cabin, inside of the blue briefcase, along with the notebook and pen. After this, the calculator can be accessed at any point during the game by pressing the Y button. Pressing the Y button again (while in the calculator) will close the calculator screen. The calculator is rarely used by some players because most (if not all) calculations involved in the Nonary Game are just simple addition.
The calculator resembles a common 4-function calculator. However, it has a few key differences. Only 3 elementary operations are supported: addition, subtraction, and multiplication. The calculator cannot divide nor support decimal points, so only integer values are supported. The calculator has two displays: the top holds an expression, and the bottom holds the evaluated answer. Both displays can display a maximum of 12 characters. Also, unlike other 4-function calculators, since the top display shows an expression, the order of operations is also implemented. For example: 7 + 3 x 8 will evaluate as 31 instead of 80. One additional function that the calculator has is the digital root function. This function will evaluate the current expression displayed and output its digital root. The calculator display can also be cleared by tapping the [CLEAR] button.