Standard Deviation Calculator

The Standard Deviation Calculator measures how spread out a set of numbers is. Paste a list of values, separated by commas, spaces, or new lines, and it returns the standard deviation along with the full set of descriptive statistics that go with it: count, sum, mean, median, mode, minimum, maximum, range, variance, and the coefficient of variation. A low standard deviation means the values cluster tightly around the average; a high one means they are scattered widely.

Standard deviation is one of the most used numbers in statistics because it puts a single figure on variability in the same units as the data itself. Two classes can have the same average test score of 75, but if one has a standard deviation of 3 and the other 18, they are very different groups. The average alone hides that; the standard deviation reveals it. It underlies confidence intervals, z-scores, control charts, risk measures in finance, and the "plus or minus" on almost any reported statistic.

The tool computes both the population and the sample version, because the choice changes the answer and is easy to get wrong. Population standard deviation divides the sum of squared deviations by N and is correct when your numbers are the entire group you care about. Sample standard deviation divides by N minus 1, an adjustment called Bessel's correction that removes the bias you get when a sample is used to estimate a larger population it was drawn from. When in doubt about a sample, the N minus 1 version is the conventional choice, and both are shown side by side so you never have to recompute.

Everything runs in your browser in plain JavaScript. Your data set is never uploaded, there is no sign-up, and the calculator keeps working with no connection once the page has loaded. It handles decimals and negative numbers, ignores nothing silently, and flags any token it cannot read as a number so a stray label or currency symbol does not quietly skew the result.