Hey,
Here is the retention rate formula:
% Retention Rate (RR) = 100 - [{S - (UT + BT)} / AE x 100]
Where:
RR = Retention Rate
S = Total number of terminations from whatever cause (this means the total number of employees that have left the company for any reason)
UT = Unavoidable Terminations (e.g., retirements or long-term illness)
BT = Beneficial Terminations (e.g., sometimes those staff who leave provide benefits to the company by virtue of leaving, such as underperformers)
AE = The average number of employees working for the company during the same period as calculated; this should be any period of 12 months.
Hope you will find this useful.
-- Priyanka