Daily Standard Deviation

Dear members,
I am trying to calculate the daily standard deviation from a column of daily percentage changes in a stocks price. What I would need is a monthly value that calculates the standard deviation of the daily stock prices in the respective month.

I have attached a simplified version of my excel sheet;
-Relevant columns are marked blue
-Column C gives the number of the month so that I can calculate the monthly return
-I would like to have the standard deviation of each month's daily return similarly to the monthly returns calculated in column H.

I would we really grateful for any help or hint how it could work!!