The following formula is involved in the calculation of covariance values from correlation coefficients:

Covariance value:
where r_{XY} is the Pearson correlation between two variables X and Y, σ_{X} is the standard deviation for X, and σ_{Y} is the standard deviation for Y.

The following references can be used to cite this covariance calculator (from a correlation coefficient):
 Soper, D.S. (2014). Covariance Calculator (from a Correlation Coefficient) [Software]. Available from http://www.danielsoper.com/statcalc 
 Cohen, J., Cohen, P., West, S.G., and Aiken, L.S. (2003). Applied Multiple Regression/Correlation Analysis for the Behavioral Sciences (3rd edition). Mahwah, NJ: Lawrence Earlbaum Associates. 