How do I calculate standard deviation in Excel for use with error bars?

Use the `STDEV.S(range)` function. Replace 'range' with the range of cells containing your data (e.g., `STDEV.S(A1:A10)`). This function calculates the sample standard deviation. If you have the entire population, use `STDEV.P(range)`. The standard deviation is then used to add 'Standard Deviation' error bars as described previously, or it's used to calculate custom error bar values.