Work days per year or the factor rate may vary every year depending on the number of weekend days. You may refer to this link for more information from The National Wages and Productivity Commission (NWPC)
Usually, we use the following factor rates on Sprout Payroll:
313 work days per year if an employee has a work schedule from Mondays to Saturdays
261 work days per year if an employee has a work schedule from Mondays to Fridays.
You may refer to this related article for more information: How Do I Set the Work Days per Year for Specific Employees?