Pew starts with the U.S. median household income.
Then Pew defined "middle class" as households that had at least two-thirds of the median income, but no more than double that amount adjusted for household size.
Based on the above definition, for a household with three people, being middle class means making between about $42,000 and $126,000. If a family of three makes less than $42,000, then it is in the lower class. If it makes more than $126,000, it is in the upper class.
Based on this definition, middle in America is shrinking.