Attrition is a reduction in force by means of resignations, retirements, and death. Turnover is defined as a change in the workforce due to accessions, quits, discharges, and layoffs. The difference is that attrition is a function of a declining workforce, whereas turnover is the function of a stable or expanding workforce.
The general measure of turnover is a percentage derived from the number of separations divided by the total number of workers on the payroll. For example, if 10,000 employees left a workforce of 5,000:
10,000/5000 = 10/5 = 2/1 = 200%