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Quick answer: enter a value, the mean and the standard deviation above — e.g. a score of 85 with mean 70 and SD 10 gives a z-score of 1.5.
Z-score calculator
Short answer: the z-score tells you how many standard deviations a value is from the mean: z = (x − μ) / σ. Enter your numbers above.
How to calculate a z-score
Short answer: how to calculate a z-score: subtract the mean from the value, then divide by the standard deviation. The calculator shows the working and interprets the result.
Frequently asked questions
What is a z-score?
A z-score measures how many standard deviations a value sits from the mean. z = 1.5 means the value is 1.5 SDs above the mean.
What is the z-score formula?
z = (x − μ) / σ, where x is the value, μ the mean and σ the standard deviation. The calculator applies it instantly.
What does a negative z-score mean?
The value is below the mean. z = −2 means two standard deviations below average.
What is a good z-score?
Depends on context — in test scores, positive is above average. Anything beyond ±3 is usually an outlier, which the calculator flags.
Can the standard deviation be zero?
No — with zero spread every value equals the mean, so a z-score is undefined. The calculator requires a positive SD.
Is this z-score calculator free?
Yes — unlimited calculations, free forever. No account, no limits. Everything runs in your browser.