I gotchu these stats come from the 2012 & 2014 PIAAC, or Program for the International Assessment of Adult Competencies (only done every 10 years so this is the latest data available but the next cycle/data set is coming soon)

18% are below level one, defined as;

The tasks at this level require the respondent to read brief texts on familiar topics to locate a single piece of specific information. There is seldom any competing information in the text, and the requested information is identical in form to information in the question or directive. The respondent may be required to locate information in short continuous texts; however, in this case, the information can be located as if the text were noncontinuous in format. Only basic vocabulary knowledge is required, and the reader is not required to understand the structure of sentences or paragraphs or make use of other text features. Tasks below Level 1 do not make use of any features specific to digital texts.

With 4% of those being actually entirely illiterate

Breakdown

Actual methodology and whatnot