Frequently Asked Questions
Common questions about the Z-Score Calculator.
What is a z-score?
A z-score tells you how many standard deviations a value is from the mean of its distribution.
How do I calculate a z-score?
Enter your raw value, the mean, and the standard deviation, and the calculator applies z = (x − μ) / σ.
What does the percentile mean?
It's the percentage of the distribution that falls below your value, based on the standard normal distribution.
Can I go the other way, from z-score to percentile?
Yes, switch to the "from z-score" mode and enter the z-score directly.
What does the shaded curve show?
It visualizes the area under the standard normal curve corresponding to your selected result (left, right, or two-tailed).
What if my standard deviation is zero or negative?
Standard deviation must be greater than zero; the calculator flags invalid entries instead of producing an error or NaN.
How accurate is the percentile calculation?
It uses a high-precision normal CDF approximation, matching standard z-tables to four decimal places.
Can I find the area between two z-scores?
Yes, the "between" option shows the area between −z and +z.
Is my data stored?
No, everything is calculated client-side in your browser.
Is it free?
Yes, completely free with no sign-up or limits.