There is no single agreed formula. Many teams combine several indicators for each hire, such as first-year performance rating, time to reach full productivity, hiring-manager satisfaction at 90 days and whether the person is still in the role after a year, then average the results by role, source or recruiter. A sales team might also track how quickly new hires reach their first target.
Speed and cost metrics show how efficient a hiring process is, but quality of hire shows whether it works. Linking outcomes back to screening scores, interview ratings and source helps identify which criteria predict success and which add little. The data takes months to mature and samples can be small, so trends across several hiring rounds are more reliable than any single figure.