What a z-score says
A z-score of 0 is exactly at the mean. A z-score of 1 is one standard deviation above the mean, while -1 is one standard deviation below. Z-scores are useful for comparing values measured on different scales.
Example
If a value is 85, the mean is 70 and the standard deviation is 10, the z-score is (85 - 70) / 10 = 1.5. The value is 1.5 standard deviations above the mean.
Common mistakes
The standard deviation must be greater than zero. Also make sure the value, mean and standard deviation are based on the same measurement scale.
When percentiles need more context
This calculator does not assume the data is normally distributed. Percentile interpretations require distribution assumptions or a separate normal-distribution model.
Last reviewed: 2026-05-17