You can calculate Attrition Rate = Number of Employees Left/Average Number of Employees *100.

Take an instance where in total employee strength is 150 as of April 1, 2015. In the same month, 20 employees left the organisation. Also, 25 new employees were hired.

Firstly we need to calculate the average employee strength. Opening balance was 150. If 20 people left and 25 people were hired, then the closing balance was 155.

Average Employee Strength for that month = {(150+155)/2=152.5}.

Finally, we will calculate attrition rate:-{ (20/152.5)*100=.1311*100=13.11}

So, we get the attrition rate for the month of April, 2015 was 13.11 percent.

You can also visit following link to see how to generate Employee Attrition Report.