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Answer: Readability formulas count syllables, words, and sentences; none takes into account the content of the document or whether you have conveyed the information clearly.

Evidence from two federal guides

• SEC handbook: no formula can tell you if you have conveyed the information clearly; by some formulas, Einstein's theory of relativity reads at a 5th grade level.
• Computerized style checkers identify the passive voice and may suggest ways to make writing more readable; take their suggestions as just that.
• CDC Clear Communication Index: readability formulas are inadequate to assess communication effectiveness; they don't consider audience, purpose, or most characteristics that contribute to clarity and comprehension.
• CDC: short words and sentences help some aspects of cognitive processing but are not sufficient to ensure clear communication.

What to do instead

The final test of whether writing meets its goal of communicating information comes when humans read it.

See also

• plain-language-test-with-readers
• plain-score-with-clear-communication-index

Claims

• id: C1 claim: No readability formula takes content into account. source: S1 evidence: "No formula takes into account the content of the document being evaluated."

• id: C2 claim: Final test is human readers. source: S1 evidence: "test of whether any piece of writing meets its goal of communicating"

• id: C3 claim: CDC calls readability formulas inadequate. source: S2 evidence: "Readability formulas are inadequate to assess communication effectiveness."

• id: C4 claim: Short words and sentences are not sufficient. source: S2 evidence: "they are not sufficient for ensuring that communication is clear and effective"

Sources

• [S1] A Plain English Handbook. U.S. Securities and Exchange Commission. https://www.sec.gov/pdf/handbook.pdf . License: US government work, public domain. Accessed 2026-11-09.

• [S2] CDC Clear Communication Index: User Guide. Centers for Disease Control and Prevention (CDC). https://www.cdc.gov/ccindex/pdf/clear-communication-user-guide.pdf . License: US government work, public domain. Accessed 2026-11-09.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted). Sources: US federal government works (GSA, GPO, SEC, CDC), public domain in the United States; third-party quotations inside those works were not used. Text was condensed and changed from the originals.

@misc{iil_note_p9s98wtc,
  author = {{iinventedlight}},
  title = {{Readability scores do not prove a text is clear}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-09},
  url = {https://iinventedlight.com/library/notes/plain-readability-formulas-limits/},
  note = {Permanent ID iil:note:p9s98wtc. License: CC-BY-4.0.},
}

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