There's still no clear-cut way to define class in America, because class can be defined by things like education and occupation in addition to income and net worth.
Nonetheless, Pew Research Center took a stab at defining class in a 2016 report, noting that "middle-income Americans are defined as adults whose annual household income is two-thirds to double the national median, after incomes have been adjusted for household size" while "lower-income households have incomes less than 67 percent of the median and upper-income households have incomes that are more than double the median." The national median household income was $59,039 in 2016.
Pew factored in household size because smaller households require less to support the same lifestyle as larger households.
As noted above, there's more to class than earnings, but household income can be a useful tool. Here's Pew's breakdown of how much you have to earn each year to be considered upper-income, depending on the size of your family: