- U1 = Percentage of labor force unemployed 15 weeks or longer
- U2 = Percentage of labor force who lost jobs or completed temporary work
- U3 = Percentage of labor force who are without jobs and have looked for work in the last four weeks (note that this is the officially-reported unemployment rate)
- U4 = U3 plus the percent of the labor force that count as "discouraged workers," i.e. people who would like to work but have stopped looking because they are convinced that they can't find jobs
- U5 = U4 plus the percent of the labor force that count as "marginally attached" or "loosely attached" workers, i.e. people who would theoretically like to work but haven't looked for work within the past four weeks
- U6 = U5 plus the percent of the labor force that count as "underemployed," i.e. part-time workers who would like to work more but can't find full-time jobs
Technically speaking, the statistics for U4 through U6 are calculated by adding discouraged workers and marginally attached workers into the labor force as appropriate. (Underemployed workers are always counted in the labor force.) In addition, the BLS defines discouraged workers as a subset of marginally attached workers but is careful not to double count them in the statistics. You can see the definitions directly from the BLS here.
While U3 is the main officially reported figure, looking at all of the measures together can provide a broader and more nuanced view of what is happening in the labor market.