Computing Regular Rate of Pay - Fixed Weekly Salary For a Fixed Workweek of More Than 40 Hours
The regular rate for an employee working a fixed workweek of more than 40 hours is half-time for hours regularly worked over 40 and time and one half for additional hours. 29 C.F.R. § 778.114(b)

Example:
An employee is paid $480 for a 48-hour week. The applicable statutory straight-time workweek is 40 hours. The employee’s rate is $10 per hour ($480 divided by 48 hours = $10). The weekly wage is $520, calculated as: $480 + [(½ X $10) X (48-40)]. For hours worked over 48, the employee must be paid time and a half, or $15 per hour, and is not eligible for half-time pay since the salary did not represent payment for hours worked in excess of 48 per week.