On the default Natural HR "Dashboard" view, there is a tile which displays the attrition rate for your organisation, like so:

As of June 2020, there two options for how this attrition rate is calculated.  You can choose between these options by going to:

Administration > Company > Settings > Dashboard settings

(The setting can be found under the Dashboard heading, labelled Attrition Calculation)

The options available are:

• Original - The system's default attrition calculation method takes the number of leavers and starters over a 12 month period, and the number of employees as of today, and uses the following calculation:
  
  (leavers x 100/(employees+starters)) = Attrition Rate
  
  For example, if a company has 100 employees today and in the last 12 months has had 10 leavers and 15 starters the calculation will be:
  
  (10 x 100/(100+15)) = 8.69%
  
  
  
• Revised - This new alternate calculation method takes the number of leavers in 12 months, and divides by the average employee of the headcount in the last 12 months ago and today.  The calculation used is:
  
  leavers / ((employees 12 months ago+employees today)/2) = Attrition Rate
  
  Using the same example of a company with 95 employees 12 months ago, 100 employees today, and 10 leavers in the last 12 months, the calculation will be:
  
  10 / ((95+100)/2) = 10.25%