Dear Sandeep..
To calculate attrition rate.. the formula is
Attrition rate = No. of resigned /Available Manpower *100
Suppose at the start of month May you had manpower of 10 and in the same month no. of resign is 1 only, so the available manpower after the end of the month may would be 9, then
Attrition rate = 1/9*100
Whereas if any new joining is there then it will be added in the current manpower and at the end on month the available manpower will get changed accordingly.
Regards,
Amit Seth.