A Z-score (also called a standard score or Z-Value) indicates how many standard deviations an element is from the mean. It allows you to calculate the probability of a score occurring within a normal distribution and compare two scores that are from different normal distributions.
Z = (X - μ) / σ
Where:
Z = Z-score
X = The value to be standardized
μ = Mean of the population
σ = Standard deviation of the population