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How can I calculate the volatility(standard deviation) of a stock price? and/or ROI (return on investment) of a stock?
ROI and volatility should be calculated over a representative period of time, for example 3 or 5 years, depending on data availability.  The ROI is simple, for example, over 5 years:- For the 5 year annualised volatility you can refer to the ESMA SRRI methodology. Box 1 (page 3)  m is the annualisation factor. Stock volatility calculated from weekly data should not be compared with volatility calculated from monthly data. Also, for reference: How to Calculate your Portfolio's Rate of Return