Z-Score Calculator
Find how unusual a value is: enter a raw value, the mean and the standard deviation to get the z-score and its percentile under the normal curve.
Enter your values
Your result
z = (x − μ) / σ. Percentile uses the standard normal cumulative distribution.
Frequently asked questions
What is a z-score?
A z-score measures how many standard deviations a value is above or below the mean. A z-score of 0 is exactly average, +1 is one standard deviation above, and −1 is one below.
How do you calculate a z-score?
Subtract the mean from the value, then divide by the standard deviation: z = (x − μ) / σ. This tells you how far the value sits from the average in standard-deviation units.
What does the percentile tell me?
The percentile is the share of a normally distributed population that falls at or below your value. A z-score of 2 is roughly the 97.7th percentile — about 2.3% scored higher.
What is a good z-score?
It depends on context. In testing and quality control, |z| ≥ 2 is typically considered unusual and z of 3 or more is often flagged. Positive scores are above average; negative are below.
Where are z-scores used?
They are used in standardized tests (SAT, ACT), statistics classes, finance and quality control — anywhere scores on different scales need to be compared.