Z-Score Calculator

What is the Z-Score Calculator?

Calculate the z-score of a value and its percentile under a normal distribution. A z-score shows how many standard deviations a value sits from the mean, which lets you compare results from different scales.

How to use the Z-Score Calculator

1. Enter your value, your mean, your standard deviation.
2. Results update automatically as you type; you can also press Calculate.
3. Read the output: z-score, percentile (below x).
4. Use the 📊 chart to see the result visually, then copy or share it.

Formula and method

Worked example

With value = 85, mean = 70, standard deviation = 10, the Z-Score Calculator returns z-score: 1.5000; percentile (below x): 93.32%.

Where the Z-Score Calculator helps in real life

You scored 82 on a test with a mean of 70 and a standard deviation of 8. A z-score of 1.5 puts you around the 93rd percentile.

A quality engineer flags parts whose measurements have z-scores beyond ±3 as possible defects.

Common mistakes to avoid

• Using the wrong standard deviation.
• Assuming percentiles apply to non-normal data.
• Dropping the negative sign for values below the mean.
• Comparing z-scores from very different populations.

💡 Practical tip: A z-score of 0 means the value equals the mean exactly.

Tips for better results

• Check that data is roughly bell-shaped before using percentiles.
• Use z-scores to compare results on different scales.
• Look into values beyond ±3 as potential outliers.

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