Execl will figure you SD for you.
execl will also give you an R^2 value (There are several different definitions of R2 which are only sometimes equivalent. One class of such cases includes that of linear regression. In this case, R^2 is simply the square of the sample correlation coefficient between the outcomes and their predicted values, or in the case simple linear regression, between the outcome and the values being used for prediction. In such cases, the values vary from 0 to 1. )
for those non-math majors out there R^2 should be as close to 1 as you can get it. I will not bore you with the detailed meaning other than to say this is also a tool to make sure your number are in the right "range" , and not seeing numbers grossly outside of that "range" based on the analysis of the quantity of data.
Below gives a pretty watered down concept of what SD is and the Excel functions
http://www.who.int/tb/surveillanceworkshop...uncertainty.htm