Computing Regular Rate of Pay - Monthly Salary Pay
To compute the regular rate for an employee who is paid monthly, the employee’s salary must be multiplied by 12 (the number of months in the year) and divided by 52 (the number of weeks in the year). This figure must then be divided by the number of hours in a workweek. 29 C.F.R. § 778.113(b)

Example:
If an employee earns $1,500 a month and has a statutory straight-time workweek of 40 hours, the employee’s regular rate would be $8.65: ($1,500 X 12)/52 = $346.15 per week; $346.15/40 = $8.65 hourly rate. If the employee worked 44 hours in a workweek, the employee would be entitled to overtime pay for four hours at time and a half: ($8.65/2 = $4.33; 4.33 + 8.65 = $12.98 overtime rate), 4 X $12.98 = $51.92. The employee’s salary for the month would be $1,551.92. The employee also might be eligible for half-time treatment if his salary represented all hours worked.