The following formula is involved in the calculation of beta values:

Beta value:
where COV_{(I,M)} is the covariance between the investment returns and the market returns, and VAR_{(M)} is the variance of the market returns.

The following references can be used to cite this beta value calculator:
 Soper, D.S. (2018) "Beta Value Calculator (Online Software)", http://www.danielsoper.com/fincalc. 
 Siegel, J. and Shim, J. (1997). "Schaum's Quick Guide to Business Formulas", New York, NY: McGrawHill. 