Standard Deviation Calculator
Paste a list of numbers and get both flavours of the standard deviation at once: the sample SD (divides by n − 1, what R's sd() and Excel's STDEV.S compute) and the population SD (divides by n, Excel's STDEV.P). The full spread summary comes with it: variance in both versions, sum of squared deviations, standard error of the mean and the coefficient of variation. No more guessing which formula an online tool used — both are shown side by side, clearly labelled. All computation happens in your browser; your data never leaves your device.
Frequently asked questions
Sample or population standard deviation — which one do I need?
Use the sample SD (n − 1 denominator) whenever your data are a sample drawn from a larger population and you want to generalize — which is almost every study and what statistics software reports by default. Use the population SD (n) only when your data include every member of the group you care about, e.g. the grades of one specific class.
How do I report the standard deviation in APA 7 format?
As SD alongside the mean: "(M = 13.7, SD = 1.42)", typically with the sample size: "N = 10". The copy button gives you the M/SD/N string directly; note that APA reports the sample (n − 1) version.
What is the coefficient of variation useful for?
CV = SD / mean × 100 expresses spread as a percentage of the mean, making variability comparable across scales and units — e.g. whether reaction times vary relatively more than error counts. It is only meaningful for ratio-scaled, all-positive data.