Given a normal distribution, by the 68-95-99 rule of thumb, about 68 percent of the data will have a Z-score on the range -1 to 1 inclusive (that is, will be between -1 and 1 standard deviations from the mean), about 95% will have a Z-score between -2 and 2, and about 99.7% will have a Z-score between -3 and 3. For more accurate values than the 68-95-99 rule, one should consult a table or calculator.

A Z-score is often looked up on a Z-table to determine what percent of the data should be within a certain range, or to find the probability that an arbitrary event occurred. On a TI-83 or TI-89, the normcdf and normpdf (for the former) or tistat.normcdf and tistat.normpdf (for the latter) can take the place of a Z-table.

When the standard deviation of the population is not known, a T-score can be used.