Z score is the number of standard deviation from the mean and is calculates using a raw score, a mean and a standard deviation. Therefore a z score of 3 would mean that the raw score (30) minus the mean (15) divided by the standard deviation (5) would show the results of 3. Percentile is usually to show a standing or position in a set of number such as using your test score to determine your position in relation to other test scores. If your test score had. Position of say 8 out of twenty you could calculate using R=P(100)/20+1 would yield a percentile of 38%