Z-Score Calculator
How many standard deviations a value is from the mean.
Result
Z-score
2.00
How it works
A z-score standardises any value: z = (value − mean) ÷ standard deviation. It answers "how unusual is this?" by counting standard deviations from the average. A positive z sits above the mean, a negative one below, and z = 0 is exactly average. Standardising makes incomparable things comparable. A height in centimetres and a test score out of 200 both become pure numbers on the same scale, so you can say which is the more extreme result relative to its own population. As rules of thumb in a normal distribution: about 68% of values fall within ±1, 95% within ±2 and 99.7% within ±3. That is why |z| above 2 is commonly called unusual and above 3 an outlier — the basis of quality control charts and anomaly detection.
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Frequently asked questions
What does a z-score mean?
It is the distance from the mean in standard deviations — positive is above average, negative below.
What z-score is unusual?
Roughly |z| > 2 covers only ~5% of a normal distribution, so it is considered uncommon.
Which z-scores count as unusual?
In a normal distribution, roughly 95% of values sit within ±2 and 99.7% within ±3. So |z| > 2 is typically flagged as unusual and |z| > 3 as an outlier worth investigating.
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