The calculation of attrition rate is fairly simple.
Identify the base date and the employee count on that day. - X
Determine the new joinings in the organisation after base date - j
Determine the resignations in the organisation after base date - r
(if you want to identify reasons related to attrition, use the Date of resignation of the employee.. however, for accurate attrition data for calculation for productivity, usage of 'Last Working Day' is preferred.
If you are not sure of the reason, use 'Last working day' as criteria)
Now, what you have is the current employee count in the organisation - Y
So, Attrition rate = r / ((X+Y)/2)
(X+Y)/2 --> averages out the employee count in the organisation during the period.
To annualize the attrition,
Annual Attrition = (12/Duration (in months))*(r/(X+Y)/2)
Hope this solves the attrition problem.
You can build your own calculator in excel using the above formula.
Regards,
Anup.