Employee Retention Rate can also be known as Employee Turnover Rate. When Employee Turnover Rate is used as a KPI, basically, you WANT some turnover, and whilst you don't want to REWARD a manager for firing people, you don't want to penalize them for the poor performers leaving. The formula to calculate annual retention rate or employee turnover rate is: Sum of staff departures over the year divided by Sum of Monthly Staff Headcount.
Regards, Autumn Jane