Z-Score Calculator

About this calculator

A z-score standardizes a value: z = (x − mean) ÷ standard deviation. It answers "how unusual is this?" on a universal scale — z = 2 means two standard deviations above the mean, z = −1.5 means one and a half below.

For roughly normal data, the 68–95–99.7 rule gives instant context: about 68% of values fall within one standard deviation of the mean, 95% within two, 99.7% within three. A z-score beyond ±2 is uncommon; beyond ±3 is rare.

Z-scores make different scales comparable: an IQ of 130 (mean 100, SD 15) and a height of 6'4" are both z = 2 — equally far from typical, in their own distributions.