Hi,
There are two practices being followed by organizations to calculate the attrition rate:
1) No of Exit in the month/(no of employees at beginning of the month+ New joinings) *100
2) No of Exit in the month/ {(no of emp at beginning+no of emp at end of month)/2} *100
In both the cases, you will find very little difference in the attrition rate. Just for your info, many MNC and big companies follows S.No 2 formula to know their attrition rate.
Regards,
Anup