By definition (of the OCDE) the middle class is the middle 3 quintiles, e.g. people who make more than the bottom 20% and less than the top 20%.

In US in 2011, that meant a household making between about 20k and 100k. So yes the middle class is huge (60% of the population by definition), and 40k is right in the middle quintile (between 38k and 62k). Not upper-middle-class, but not "minimum wage working class" either.

http://en.wikipedia.org/wiki/Household_income_in_the_United_...