In September 2018, the civilian labor force was just 62.7% of the civilian noninstitutional population (161.958 million X 100 / 258.290 million = 62.7).
Not seasonally adjusted, this yielded an unemployment rate of 3.6% (5.766 million unemployed X 100 / 161.958 million = 3.56, before rounding up).
Unfortunately that's only because the labor force shrank by 8.5 million since 2008. The labor force then averaged 66% of the civilian noninstitutional population, not today's 62.7%.
Taking 66% of September 2018's civilian noninstitutional population means a labor force of 170.5 million instead of the not quite 162 million we've actually got. Where'd all those 8.5 million go? New Zealand?
Add 'em back in on both sides of the equation, both to the size of the labor force and to the unemployed, because they are obviously not working, and unemployment soars to . . . 8.4% (14.266 million unemployed X 100 / 170.458 million = 8.36).
All this labor slack is the reason wages fail to go up at rates of 3-4% as in previous recoveries.
It's not a tight labor market.