Z-score calculator

The z-score counts standard deviations between the value and the mean: z = (x − mean) ÷ sd. The percentile next to it is the share of a normal distribution sitting at or below that score, computed from the error function in this tab. The engine assumes the standard deviation you enter describes the whole population. When it is a sample estimate instead, treat the percentile as a rough figure, and note that none of this is a hypothesis test: no p-values, no sample sizes, no tail comparisons. A z-score of 0 always lands on the mean, and a standard deviation of zero is refused because a distribution with no spread cannot score anything.