Z-Score Calculator 🔒 Your data never leaves your browser.
Calculate a z-score, or convert a z-score back to a raw value.
About this tool
A z-score measures how many standard deviations a value sits above or below the mean of its distribution. A z-score of 0 means the value is exactly average; a positive z-score means it’s above average, and a negative one means it’s below. Because the calculation only depends on the mean and standard deviation of the surrounding data — not the original units — z-scores let you place completely different measurements on the same scale.
That’s what makes z-scores so useful for comparison: a test scored out of 100 and a different test scored out of 40 can’t be compared directly by raw points, but once each score is converted to a z-score, you can see which one was actually more impressive relative to its own distribution. Switch to "Z-score → Value" to go the other way — enter a target z-score along with a mean and standard deviation to find out what raw value would produce it. Everything is calculated instantly in your browser, and nothing you type is ever sent anywhere.
Frequently asked questions
What counts as an "unusual" z-score?
As a common rule of thumb in a roughly normal (bell-curve) distribution, a z-score beyond ±2 is often considered notably unusual (only about 5% of values fall further from the mean than that), and beyond ±3 is quite rare (about 0.3%). These thresholds come from the properties of the normal distribution specifically — if your underlying data isn't actually normally distributed, these rough percentages don't strictly apply, though the basic idea that "further from zero means further from typical" still holds.