29 C.F.R. § 778.209 : Labor — REGULATIONS RELATING TO LABOR — WAGE AND HOUR DIVISION, DEPARTMENT OF LABOR — STATEMENTS OF GENERAL POLICY OR INTERPRETATION NOT DIRECTLY RELATED TO REGULATIONS — OVERTIME COMPENSATION — PAYMENTS THAT MAY BE EXCLUDED FROM THE "REGULAR RATE" —BONUSES — Method of inclusion of bonus in regular rate

(a) General rules. Where a bonus payment is considered a part of the regular rate at which an employee is employed, it must be included in computing his regular hourly rate of pay and overtime compensation. No difficulty arises in computing overtime compensation if the bonus covers only one weekly pay period. The amount of the bonus is merely added to the other earnings of the employee (except statutory exclusions) and the total divided by total hours worked. Under many bonus plans, however, calculations of the bonus may necessarily be deferred over a period of time longer than a workweek. In such a case the employer may disregard the bonus in computing the regular hourly rate until such time as the amount of the bonus can be ascertained. Until that is done he may pay compensation for overtime at one and one-half times the hourly rate paid by the employee, exclusive of the bonus. When the amount of the bonus can be ascertained, it must be apportioned back over the workweeks of the period during which it may be said to have been earned. The employee must then receive an additional amount of compensation for each workweek that he worked overtime during the period equal to one-half of the hourly rate of pay allocable to the bonus for that week multiplied by the number of statutory overtime hours worked during the week.
(b) Allocation of bonus where bonus earnings cannot be identified with particular workweeks. If it is impossible to allocate the bonus among the workweeks of the period in proportion to the amount of the bonus actually earned each week, some other reasonable and equitable method of allocation must be adopted. For example, it may be reasonable and equitable to assume that the employee earned an equal amount of bonus each week of the period to which the bonus relates, and if the facts support this assumption additional compensation for each overtime week of the period may be computed and paid in an amount equal to one-half of the average hourly increase in pay resulting from bonus allocated to the week, multiplied by the number of statutory overtime hours worked in that week. Or, if there are facts which make it inappropriate to assume equal bonus earnings for each workweek, it may be reasonable and equitable to assume that the employee earned an equal amount of bonus each hour of the pay period and the resultant hourly increase may be determined by dividing the total bonus by the number of hours worked by the employee during the period for which it is paid. The additional compensation due for the overtime workweeks in the period may then be computed by multiplying the total number of statutory overtime hours worked in each such workweek during the period by one-half this hourly increase.

29 C.F.R. §778.209