I think there are different methods you can use, but we calculate rolling turnover by taking the total # of terms for the 12 months divided by average headcount for the same period. For example, if we had a total of 20 terms over 12 months and our average headcount was 200, then rolling turnover is 10% (yeah, I wish).
The spreadsheet we use has our numbers by month to include actual headcount for the month, actual terms for the month (we do a total and also breakdown by voluntary/involuntary), then average headcount, and our turnover calcs to include rolling and annualized. Below are the fields we use to track by month. Hope this helps.
Involuntary Number of Terms
Voluntary Number of Terms
Total Terms % of HC
Month End Headcount
Annualized Monthly Involuntary Turnover (%)
Annualized Monthly Voluntary Turnover (%)
Annualized Monthly Total Turnover (%)
Average Headcount (12 month period)
Total Involuntary terms (12 month period)
Total Voluntary terms (12 month period)
Total terms (12 month period)
Annual Rolling Involuntary Turnover (%)
Annual Rolling Voluntary Turnover (%)
Annual Rolling Total Turnover (%)