The following formula is involved in the calculation of Cohen's d effect size values for ttests:

Cohen's d effect size for a ttest:
where x_{1} and x_{2} are the means of group 1 and group 2, and σ_{1}^{2} and σ_{2}^{2} are the variances of group 1 and group 2.

The following references can be used to cite this effect size (cohen's d) calculator for a student ttest:
 Soper, D.S. (2014). Effect Size (Cohen's d) Calculator for a Student tTest [Software]. Available from http://www.danielsoper.com/statcalc 
 Cohen, J. (1988). Statistical Power Analysis for the Behavioral Sciences (2nd Edition). Hillsdale, NJ: Lawrence Earlbaum Associates. 