Source: U.S. Bureau of Labor Statistics  

Release: Characteristics of Minimum Wage Workers  

Units:  Percent of Total Paid Below Minimum Wage, Not Seasonally Adjusted

Frequency:  Annual

Wage and salary workers are workers age 16 and older who receive wages, salaries, commissions, tips, payments in kind, or piece rates on their sole or principal job. This group includes employees in both the private and public sectors.
Workers paid by the hour are employed wage and salary workers who report that they are paid at an hourly rate on their job. Estimates of workers paid by the hour include both full-time and part-time workers unless otherwise specified. All self-employed workers are excluded whether or not their businesses are incorporated.

The estimates of workers paid at or below the federal minimum wage are based solely on the hourly wage they report (which does not include overtime pay, tips, or commissions). Salaried workers and other nonhourly paid workers are also excluded. It should be noted that some respondents might round hourly earnings when answering survey questions. As a result, some workers might be reported as having hourly earnings above or below the federal minimum wage when, in fact, they earn the minimum wage.
For more information see https://www.bls.gov/cps/earnings.htm#minwage