Keep in mind that these people vote for reality show actors - we are screwed.

That's just for adults, for children the data ranks the States against one another rather than giving an average illiteracy percentage like with adults. Here's a top tier link on the topic: https://www.thinkimpact.com/literacy-statistics

Original source: https://www.oecd.org/en/publications/do-adults-have-the-skills-they-need-to-thrive-in-a-changing-world_b263dc5d-en/full-report.html

Another review of the source material for good measure: https://www.crossrivertherapy.com/research/literacy-statistics

I don't use algebra, calculus etc. in my daily life, but I sure speak and read English every single day regardless of what I am doing. 

Not sure why you'd expect everyone to know high level math when we don't use it.

Prose literacy: Can you read a newspaper article and understand the main points?

Document literacy: Can you fill out a job application or interpret a map?

Quantitative literacy: Can you balance a checkbook or understand a chart with multiple data points?

Someone reading at a 6th-grade level can handle most day-to-day reading tasks. They can read their prescription bottles, follow basic instructions, and understand straightforward news articles.

What they struggle with is synthesizing complex information from multiple sources, drawing inferences from technical documents, or navigating websites with multiple layers of information.

Checking out readability formulas, like the Flesch‑Kincaid grade level formula, should help to clear things up a bit. But in general, a 6th grade reading level is where concrete language, moderate sentence length, and single step reasoning all align. Adding abstract vocab, multiple embedded clauses, or deep cultural references push text above a 6th grade level.