I did look it up earlier, but I didn't find the source on that specific statistic or what it meant, the stats I found didn't match it. I was hoping somebody here knew where it was from or what it meant.
This, from 2021, came up and places US illiteracy levels at 4% or 18%, depending on whether you restrict it to total illiterates or not: https://www.wyliecomm.com/2021/08/whats-the-latest-u-s-literacy-rate/
Several others put it at 18%, too.
Looking again, though, there is this, which is consistent with the stat in that image:https://www.thinkimpact.com/literacy-statistics/
It seems to be referencing this: https://web.archive.org/web/20200730223012/https:/nces.ed.gov/datapoints/2019179.asp
And this other reference which ThinkImpact cites (which says its from 2023) also seems to be relying on that same NCES data from the 2010s: https://worldpopulationreview.com/state-rankings/us-literacy-rates-by-state
Interestingly, NCES themselves get the 21% number this way:"Four in five U.S. adults (79 percent) have English literacy skills sufficient to complete tasks that require comparing and contrasting information, paraphrasing, or making low-level inferences—literacy skills at level 2 or above in PIAAC (OECD 2013). In contrast, one in five U.S. adults (21 percent) has difficulty completing these tasks (figure 1). This translates into 43.0 million U.S. adults who possess low literacy skills: 26.5 million at level 1 and 8.4 million below level 1, while 8.2 million could not participate in PIAAC’s background survey either because of a language barrier or a cognitive or physical inability to be interviewed. These adults who were unable to participate are categorized as having low English literacy skills, as is done in international reports (OECD 2013), although no direct assessment of their skills is available.
Meaning that the 21% number includes not only people who are confirmed fully illiterate (4.1% were confirmed below level 1 literacy), but also those who are at level 1 literacy (12.9%) and can read some,
and those who could not participate (4.0%), which non-English speakers included in this last group. As best as I can tell, this is the source of the 21% statistic.