Effect sizes are highly relevant in quantitative research. They facilitate the comparison an quantitative synthesis of scientific studies. The main objective of this report is to present: a) a brief summary of the formulas used for conversion between the three main effect sizes used in the meta-analysis: the correlation coefficient, the standardized mean difference and the odds ratio; and b) the Rapid Effect Size Converter for Meta-Analysis (rESCMA), a open-source and browser-based app for efficiently converting and bulk-converting effect sizes and their variances based on the formulas proposed in this paper. In addition, a table summarizing the formulas is presented for easy accessibility and use.