This Campbell methods policy note on converting between effect sizes sets out current Campbell Collaboration policy on calculating a common effect size from different metrics for use in meta‐analyses in Campbell systematic reviews of intervention effects. It is intended to serve as a point of reference for Campbell authors and editors in both the design and production phases of Campbell reviews of intervention effects, to ensure that protocols and full reviews are prepared in line with collaboration policy. In order to provide context for the current policy, the Note is prefaced with a brief introduction to the topic and appended with a list of additional resources, tables of formulas and worked examples. It is not intended to provide comprehensive guidance or a tutorial on how to apply methods in Campbell or other systematic reviews