- U1: This is the proportion of the civilian labor force that has been unemployed for 15 weeks or longer. This unemployment rate measures workers who are chronically unemployed. During business-cycle expansions, this rate captures structural unemployment. However, during lengthy business-cycle contractions, this rate is also likely to include a significant amount of cyclical unemployment. U1 tends to be relatively small, in the range of 1-2 percent.
- U2: This is the proportion of the civilian labor force that is classified as job losers (workers who have been involuntarily fired or laid off from their jobs) and people who have completed temporary jobs. During business-cycle expansions, this rate is likely to capture some degree of frictional unemployment. However, during business-cycle contractions, this rate is most likely to consist of cyclical unemployment. U2 is larger than U1, but still remains substantially less than the official unemployment rate (U3).
- U3: This is the official unemployment rate, which is the proportion of the civilian labor force that is unemployed but actively seeking employment.
- U4: This is the official unemployment rate that is adjusted for discouraged workers. In other words, discouraged workers are treated just like other workers who are officially classified as unemployed, being included in both the ranks of the unemployed and the labor force. It is technically specified as the proportion of the civilian labor force (plus discouraged workers) that is either unemployed but actively seeking employment or discouraged workers. The addition of discouraged workers generally adds a few tenths of a percentage point to the official unemployment rate.
- U5: This augments U4 by including marginally-attached workers to the unemployment rate calculation. Marginally attached workers are potential workers who have given up seeking employment for various reasons. One of these reasons is that the workers believe such effort would be futile, which places them in the discouraged worker category. Those who have other reasons for not seeking employment are placed in the broader marginally-attached workers category. The addition of marginally-attached workers adds a few more tenths of a percentage point to the official unemployment rate.
- U6: This augments U5 by including part-time workers to the unemployment rate calculation. The addition of part-time workers adds a full 2-3 percentage points to the official unemployment rate. This measure of unemployment is perhaps the most comprehensive measure of labor resource unemployment available.
As you can see the U6 is significantly higher than U3. The monthly numbers are available at the Bureau of Labor Statistics.