The middle class is the middle three household income quintiles.

From the study:

There’s no single definition of the middle class, but one of the most go-to benchmarks is Pew Research’s household income percentile ranges for economic classes, which go as follows:

• Lower-middle class: 20th - 40th percentile
• Middle class: 40th - 60th percentile
• Upper-middle class: 60th - 80th percentile

Based on these percentile ranges, America’s “middle class” households fall into three main income tiers:

• Lower-middle class: $30,001—$58,020
• Middle class: $58,021—$94,000
• Upper-middle class: $94,001—$153,000