A standard deviation is a statistical tool that tells you roughly how far, on average, each number in a list of data values varies from the average value or arithmetic mean of the list itself. In Excel, we can use the STDEV function to provide an estimate of a set of data's standard deviation.
The function assumes that the entered numbers represent only a small portion or sample of the total population being studied. As a result, the STDEV function does not return the exact standard deviation. For example, for the numbers 1 and 2, the STDEV function in Excel returns an estimated value of 0.71 rather than the exact standard deviation of 0.5.
Practical Uses of the STDEV Function
Even though the STDEV function only estimates a standard deviation, the function still has its uses when only a small part of a total population is being tested.
For example, when testing manufactured products for conformity to the mean — for measures like size or durability — not every unit is tested and this forms an estimate of how much each unit in the entire population varies from the mean.
To show how close the results for STDEV can be to the actual standard deviation, in the image above, the sample size used for the function was less than onethird of the total amount of data yet the difference between the estimated and actual standard deviation is only 0.02.
Using the STDEV Function in Excel
A function's syntax refers to the layout of the function and includes the function's name, brackets, comma separators, and arguments. The syntax for the Standard Deviation function is as follows:
= STDEV ( Number1, Number2, ... Number255)
 Number1: This is required. It can be actual numbers, a named range, or cell references to the location of data in a worksheet. If cell references are used, empty cells, Boolean values, text data, or error values in the range of cell references are ignored.
 Number2, ... Number255: This is optional. Up to 255 numbers can be entered
An Example of the STDEV Function
For our example, the sample of data used for the function's Number argument is located in cells A5 to D7 — we will be finding the data's standard deviation. For comparison purposes, the standard deviation and the average for the complete data range A1 to D10 is included.
If you're using Excel 2010 or later, you'll need to type out the formula manually.
Follow these steps to complete the task and calculate the information using the builtin function:

Click cell D12 to make it the active cell — this is where the results for the STDEV function will be displayed

Type =STDEV(A5:D7) and press Enter.

Notice the value in D12 change to 2.37 —This new value represents the estimated standard deviation of each number in the list from the average value of 4.5
For older versions of Excel, you can type the formula manually or click in D12 and then open the visual data selector via Formulas > More Functions > STDEV.