The formula, a worked example, and how to read it as a percentile.
Written & reviewed by Dackohn; last reviewed October 2026.
To calculate a z-score, subtract the mean from your value and divide by the standard deviation: z = (x − μ) / σ. The z-score says how many standard deviations a value sits from the mean — positive above, negative below. Example: x = 85, μ = 70, σ = 10 gives z = (85−70)/10 = 1.5. Use the population mean and SD; with only a small-sample estimate, the t-distribution is more appropriate.
z = (x − μ) / σ — value minus mean, divided by standard deviation.
A test has μ = 70 and σ = 10. A score of 85 gives z = (85−70)/10 = 1.5. A score of 55 gives z = −1.5.
Look it up in a z-table or use the normal distribution calculator. z = 1.5 is about the 93rd percentile. Roughly 68% of values have |z| < 1 and 95% have |z| < 2.
=STANDARDIZE(85,70,10) returns 1.5.
Do it instantly: the z-score calculator gives the z-score and its percentile in one step.