The basic formula for calculation of Attrition is as below:
(No. of employees attrited in the year)/ (Average headcount for the year);
Where, average headcount for the year = (opening headcount + closing headcount ) / 2.
If you are calculating attrition for a particular month, then please consider the relevant figures ( no. of attrites, average headcount etc. ) for that month.
However, many organisations these days, play around with these numbers, to project lower attrition figures...a few things that are normally done are as below:
1. If closing headcount > average headcount, then closing headcount is taken as the denominator
2. Employees who attrite in the 1st 6 months are not included, while calculating the no. of attrites etc.
Hope this helps!