Z-Score Calculator

This z-score calculator tells you how many standard deviations a value sits above or below the mean, and converts that into a percentile and probability on the normal distribution. The z-score, also called the standard score, is one of the most useful ideas in statistics because it puts any value onto a common scale: subtract the mean and divide by the standard deviation, and you get a number that says exactly how unusual the value is, regardless of the original units. A z-score of 0 is right on the mean, a positive score is above it and a negative score below, with most values in normal data falling between minus 3 and plus 3. You enter the value, the mean and the standard deviation, and the calculator returns the z-score together with the percentile, the share of a normal population that falls below your value, and the probabilities of being below and above it. This is exactly what you need for standardised test scores, quality control, comparing results measured on different scales, and reading statistical tables. The percentile uses an accurate approximation of the normal distribution, so it matches printed z-tables closely. Remember the percentile assumes the data is roughly normal. Enter your value, mean and standard deviation to see the z-score and percentile straight away.