The regular rate of pay is the base on which time and a half is calculated. The Department of Labor states that it "includes all remuneration for employment except certain payments excluded by the Act itself"; discretionary bonuses are among the exclusions. For an employee on one hourly wage with no extra pay, it equals that wage. Differentials and promised bonuses make it higher.
Formula
regular rate = total remuneration for the week ÷ total hours worked
overtime premium = ½ × regular rate × overtime hours
Total remuneration here includes straight-time pay for all hours, including overtime hours, which is why only half the regular rate is added for each overtime hour.
Example
An employee earns $20 an hour, works 48 hours and receives a $100 attendance bonus. Total remuneration is 48 × $20 + $100 = $1,060, so the regular rate is $1,060 ÷ 48 = $22.08. The premium for 8 overtime hours is 8 × $22.08 ÷ 2 = $88.33, and weekly pay is $1,060 + $88.33 = $1,148.33.
See the full method in how to calculate overtime pay or use the overtime calculator.