That's incorrect.

First determine the sample mean and then use the mean to find the sample standard deviation. The sample mean and sample standard deviation are given by the formulas

x overbar equals StartFraction Upper Sigma x Subscript i Baseline f Subscript i Over Upper Sigma f Subscript i EndFraction equals StartFraction x 1 f 1 plus x 2 f 2 plus ... plus x Subscript n Baseline f Subscript n Baseline Over f 1 plus f 2 plus ... plus f Subscript n EndFractionx=ΣxifiΣfi=x1f1+x2f2+...+xnfn f1+f2+...+fn                                           and                          s equals StartRoot StartFraction Upper Sigma left parenthesis x Subscript i Baseline minus x overbar right parenthesis squared f Subscript i Over left parenthesis Upper Sigma f Subscript i Baseline right parenthesis minus 1 EndFraction EndRoots=Σxi−x2fiΣfi−1

where

x Subscript ixi

is the midpoint or value of the ith​ class,

f Subscript ifi

is the frequency of the ith​ class, and n is the number of classes.