Employers are required through the Fair Labor Standards Act (FLSA) to pay employees not less than one and one‑half times their regular rate of pay for all hours worked in excess of 40 in a workweek, unless the employees are otherwise exempt.
A workweek is a fixed and regularly recurring period of 168 hours — seven consecutive 24-hour periods. It may begin on any day and at any hour of the day.
Earnings may be determined by a variety of methods (piece-rate, hourly wage, salary, or commission), but in all cases the overtime pay must be computed on the basis of the average hourly rate derived from these earnings. This is calculated by dividing the total pay for employment in any workweek (except exclusions) by the total number of hours actually worked. Payments which are not part of the regular rate include pay for expenses incurred on the employer’s behalf, premium payments for overtime work or the true premiums paid for work on Saturdays, Sundays, and holidays, discretionary bonuses, gifts and payments in the nature of gifts on special occasions, and payments for occasional periods when no work is performed due to vacation, holidays, or illness.
If an employee in a single workweek works at two or more different types of work for which different rates have been set, the regular rate for that week is the weighted average of such rates (the earnings from all such rates are added together and the total is divided by the total number of hours worked at all jobs).
There are some classes of workers who are exempt from overtime rules. For more information on the federal requirements, see http://www.dol.gov/compliance/guide/minwage.htm or http://www.dol.gov/whd/regs/compliance/whdfs23.pdf.
Keep in mind many states have stricter guidelines that the Fair Labor Standards Act. To find rules that apply in your state, see http://www.dol.gov/whd/minwage/america.htm#.UPGlmW9EHi0.
Make sure you are in compliance when it comes to overtime rules to prevent the assessment of penalties.