Z-Score Calculator

Find the z-score (standard score) of a value — how many standard deviations it sits above or below the mean. Calculates in your browser.

Z-score

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A z-score measures how many standard deviations a value sits from the mean:

z = (x − μ) ÷ σ

A positive z is above the mean, negative is below, and zero is exactly on it. Standard deviation must be greater than zero. All math runs locally in your browser.

What this does

The z-score calculator finds the standard score of a value — how many standard deviations it lies above or below the mean of its distribution. It is the standard way to compare values from different scales.

How to use it

  1. Enter the value you are scoring.
  2. Enter the mean of the distribution.
  3. Enter the standard deviation.
  4. Read the z-score.

How it works

Subtract the mean from the value, then divide by the standard deviation. The result is in standard-deviation units, so it has no units of its own.

z = (x − μ) ÷ σ

Understanding your result

A positive z-score is above the mean, a negative one below, and zero is exactly average. As a rule of thumb, roughly 68% of values fall within ±1, 95% within ±2, and 99.7% within ±3, so a z-score beyond ±2 or ±3 marks an unusual value. Z-scores let you compare results measured on completely different scales.

Example

A score of 80 in a set with mean 70 and standard deviation 5 has a z-score of +2.

Sources & methodology

Last updated .

Frequently asked questions

What does a z-score tell you?

It tells you how many standard deviations a value is from the mean. Positive is above average, negative is below, and the size shows how unusual the value is.

What counts as a high z-score?

About 95% of values lie within ±2 and 99.7% within ±3, so a z-score beyond ±2 or ±3 is uncommon.

Why standardize a value?

A z-score removes the units and scale, so you can compare values from different distributions on equal footing.

Is my data stored?

No. Everything is calculated locally in your browser.