About this calculator
What a z-score tells you
A z-score counts how many standard deviations a value is from the mean. Positive means above the mean, negative means below, and 0 means exactly at the mean. Because z-scores have no units, they let you compare values from different scales, such as a statistics exam and a biology exam.
Unusual values
A common rule of thumb calls a value unusual when its z-score is below −2 or above 2, meaning it is more than 2 standard deviations from the mean.
Sample or population?
The arithmetic is the same either way. Use x̄ and s for a sample, or μ and σ for a population.