The calculation varies from company to company. Ideally, it should be a sum of all employees who left during the calendar month (irrespective of reason or voluntary/involuntary attrition).
The calculation of Attrition % is: (Total attrition/avg headcount) * 100. Employee Turnover ratio will be the number of employees leaving per 100 employees. So if there are 5 employees leaving in a 50-member company, the ratio would be 1:10.
Hope that helps.
krish
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