How it works: The calculator above takes the place of the traditional textbook table. The calculator can be used in two ways. To find Prob<Z for a Z score, enter a value in the "Area left of" box and hit "Return". The answer is given in red in the "=" box. To find the Z score for a probability, enter a value under in the "=" box and hit "Return". The Z score is given in the "Area left of" box. When thinking in terms of Z scores, you should use 1 as the standard deviation and 0 as the mean.