According to this source, 50% of American adults can’t read an 8th grade book.  Since the 50th percentile of American IQ is average IQ; about 100 (U.S. norms; 97 U.S. white norms), then on the IQ scale, 8th grade reading level = IQ 100 (97 on U.S.white norms).

More than 20% of U.S. adults read below the fifth grade level.

Since the 20th percentile of Americans is IQ 88 (U.S. norms; 84 on white norms), then on the IQ scale, 5th grade reading level = IQ 88 (84 on white norms).

From here, we can make simple linear formulas converting reading levels to IQ equivalents:

U.S. norms:

IQ  = 68+4(reading level)

U.S. white norms:

IQ = 62.333+4.333(reading level)